What to do about this situation about my sister?

So my sister is 26 and she got pregnant with her boyfriend 3 or 4 years ago. We're an asian Muslim family and her boyfriend is black. She's committed huge sins and its torn our family apart. My Dad was extremely angry when he found out and disowned her and no one really talks to her, apart from my Mum. When I had found out it was a year later that I found out she had a baby, and no one in my family even told me anything or I didn't even hear about it. I discovered it on my own on my mums phone when I saw a photo and I was very shocked. I couldn't believe it. I didn't confront anyone about it it and then my Mum finally decided to tell me the news after a few months.

Since then I slowly stopped talking to her and I was just so shocked and also angry because she didn't tell me and apparently my family didn't tell me because they didn't want the same thing to happen to me. I wasn't in contact with her at all, I just didn't bother with her anymore and also I just don't know what to say anymore. I just don't really care anymore to talk with her. Before she got pregnant she was already living out for uni, and when she graduated she decided to leave home and move in to her own place. My dad didn't want her to leave but I think that's fine she likes to be independent and if she wanted to move out then it's her choice. Also, when she was graduating she felt the pressure to get married and she didn't want to get married to someone she didn't want.

As the younger sister, I always wanted that big sister who was really close and loving like other families do but our relationship wasn't like that, like we were close but not that close, and sometimes she would be rude and negative towards me and swear and just not treat me very nice. Sometimes she was nice and she would help me with things and take me out but I feel like she could've been nicer in general. Just in general, she has a short temper and can have like a -----y attitude and scream and swear when she's angry. I just wouldn't like the way she is and acts sometimes. She just didn't make a big effort as my big sister back then. She's still an aggressive angry patient person. But I understand that's how she is because my dad and older brother also tend to have anger problems.

So when she was living out we would message and talk sometimes, she would visit whilst at uni until she got pregnant, she hardly visited. When she had a baby, she visited a few times over the years but secretly as my mum wanted to see her but when she came I didn't talk to her at all and just avoided her because I just felt like I didn't know what to say and it was awkward for me.

We just don't have a relationship. It's just been soo long, like over 3 years we haven't spoken or seen each other.
Honestly I just don't care anymore like she's not in my life anymore, she's the one who left and made her own choices. She has a new life and I just don't know her anymore. I've become used to being by myself and like I wish I had a big sister but I don't really have one anymore and since it's been so long I'm just not bothered anymore, I've actually been really focusing on myself and living my life and haven't given much thought to her. I don't hate her but I don't even know if I like her. I don't know, everything's just changed now, and what she's done has affected the family especially my dad, he can't sleep at night and has a lot of tension every day and takes antidepressants sometimes. He's heartbroken.

At the end of the day she is still my sister and even though I don't care so much, at the same time I just feel a bit bad and sorry for her because of what's happened and it's been difficult for her. But this is the life she chose, she made those choices and I'm sure at that time she realised what she was doing was so wrong, how cant she not know that and know the consequences. I don't know why she did it. But I know she's always wanted to be a mother and maybe she got desperate and just did it. I don't even know if her boyfriend/partner is even Muslim or not, apparently he is converted but she or her BF could be lying. And she even had another baby a couple months ago and she still isn't married and it might be due to financial reasons. But I feel like since she's got lots of non Muslim friends and she lives out she may have been badly influenced into doing wrong or tricked. Who knows. I don't know her side of the story fully or like her thoughts. She should've told someone if she wanted to be with her BF, and I'm fine with him being a different race but my parents certainly wouldn't approve of marrying outside our culture. But I think she went about things the wrong way.

Maybe she truly made a big mistake, and if she cared she would ask for forgiveness and pray etc, but it seems like she doesn't even care and she thinks she's not done anything wrong from what I've been told when my brother recently went to visit her they had a discussion but she would be like stubborn and say NO to things with an attitude.
And just thinking about this situation, I just don't really know what to think or say about it. Even though my family isn't very religious except for my parents, I do still care about wanting to be a better Muslim and even though i myself commit sins, I wouldn't ever commit major sins like sex, alcohol etc.

I just never thought something like this would ever happen in my family, and it's so sad that my dad just disowned her like that and I wish it wasn't like this. Like if my mum still has that love and care for her why can't my dad?
Growing up our parents never really taught anything about these kind of things, like about boys, sex, they're all taboo subjects and weren't spoken about. But still it's not an excuse to not know what's haraam and not.

I know we all make mistakes, none of us are perfect. But we cant change the past, we can only focus on the present and future.

Now recently my sister got my number and messaged me like 'hi happy new year' and I said the same back. She thinks I don't care about her anymore, which sad to say but honestly I don't care that much anymore and obviously she's been non existent for me. But since messaging me it seems like she's trying to amend our relationship. And I've been thinking about this situation and the future etc. And that I guess deep down I do care a little bit.

But honestly, I just don't know what to do about this situation. Like she's done alot of wrong regarding her having kids before marriage, and the way she's gone about things and the fact that it seems like she doesn't care about being a Muslim anymore. I feel like she didn't care about anyone else and only did what she wanted.

I just need some advice and guidance on how to go about this situation. If you were in this situation, what would you do? Would you talk to her, or limit how much you would say. Should I try to build a normal relationship with her again?

What should I say to her? How can I help her? What should I ask her?

I feel like maybe I should remind her what she's done wrong and the consequences, and advise her so that she changes and does better in the future? Get her on the good side?
If I should, how can I word this message, without sounding like she's being forced or just told what to do without good reason, and make her realise her mistakes and to change.

If I say things to her in the right way maybe she'll listen and care. What advice would you have for me and for my sister?

Islamically and just being a decent human, I just want to know what the right thing is for me to do and say.

I would appreciate any advice and support. Sorry for this very long post. I just wanted to let out my thoughts and sort of vent and also ask for advice.
Thank you.