sister wants to marry a guy who parents don't agree with!

asalamualikum brothers and sisters i need your help urgently!
from last year my sis has brought this converstaion up that she wants to marry aguy from different cast, we are a gujerati family and this guy is pakistani,the way the story goes...

she has just turned 24 and he has turned 30/31 not sure,he is from down south london,and we are up north, now speaking of the past, she has known him sinc she was 17/18 but has had boyfriends in between thinkin that this guy was not meant for her,now last year my sister went umrah with my brother n parents and the day before she went he contacted her n said i cant forget you and cant let go of you,i don't know what force had got into her that she kept talking to him after she come back from umrah and after a few months last year january time,she told my parents she wants to marry him and no 1 else, altogether this became a really stressful week day then week and months,i couldn't even concentrate at my studies my parents dont want us to marry in a different cast and this is all because he wants to keep his parents respect, i understand all this like if we were in my dads shoes as a parents you would probably think this way too, but i am not understanding my sis now,i was there for her as a sis and she opened up to me,these days and weeks were really awkward days and you could just feel the tension in the air everytime you would enter the house,i would always pray to allah help us out in these difficult times, me and my parents sat with her a few different nyts when she would approach my parents and talk about the guy and we sat and listened,and i said to my dad to calmly explain2her and he did,she lisened carefully-my dad explained in a calm manner, that we want wats best for you all,my parents wouldnt accept this,they are highly respected,and everyone around us even family and distance relatives ar just waiting for something o go wrong in this family and they would start talking etc,my sister lisented2 parents but still she wanted2 marry the guy n when she used to say this to my parents my mum would start crying and dad would get frustrated and angry,during these days my sis seemed really upset,down and depressed we had to cheer her up most of times but in the end she would still be feeling like this like its the end of the world,my sister approached a mufty for hope of an answer,and we know this mufty realy wel as he was a neighbour to us,my dad explained to him even he knows my grandparents who were islamic and said listen to your father he is doing the right thing keeping his parents respect and values. my parents approached a molvi and he said same, and then approached a kari saab(like another molvi) and he looked at her date of birth and his and said to my parents if she does marry him no1 would even bother feeding her etc . things only calmed down when she told him whats hapening and he stopped contact for 2weeks and she thought ok i will start afresh now, these 2 weeks passd and she seemd like a free bird flying high and as if a flower just blossemed into different colours. then after 2 weeks he started contacting her and she became the same again.
about the guy-he has 1 sis and mum, we dont know anything of his family only what my sis has told us, but taken this in mind it could not be true, they live in a flat, he doesnt have a job at the moment,he has gone to pakistan from march/april time and has yet not come back,they have been contacting each other and talking over phone but at the moment he has cut it down to maybe 1 /2 txt messages a week, my sister seems to be getting crazy over this guy, he hasnt been in touch so she has been looking depressd and upset and would cry, and we would know its something to do with this guy. yesterday was shabe beraat she had kept a roza and it was time for iftar,azaan had finished and she had not yet picked a kajoor up and had been sitting with a small face, my dad said eat your kajoor and she slowly took it to her mouth,and when my dad went for namaz she told my mum she had rang my mums parents who are back home,and we don't know what she said to them yet,but my mum didn't tell her parents about this and didnt want them to worry,my dad came after namaz again and my sister said it again and my dad started shouting saying what did you tell them,and my mum got upset thinking wat has she told them ans my dad was angry. we don't know how much this guy loves my sis, they have only met 3 tyms since the time she knew him from 17/18. we all try and make her understand but she keeps going back to square1, my dad would marry her if we knew the family well but we know nothing and only from talking over the phone their relationship has been increased,and we dont know what he's filling her ears with,he doesnt seem to be keeping in touch that much at the moment,and she seems to be the one who is in pain,feeling down and depressed, all this seems 2 me like its one sided,we dont know wats goin on in that guys head,my dad wants wats best for us, he has given us more freedom then wat other parents give to their children and mashallah we have bin on umrah and go on outings and trips together. i know my parents are hurting inside we all are, they get upset and emotional and mums usually crying about this,and so is my younger sister and me, but i dont think he is hurting inside or his family,we dont know how much he loves her! this guy haas not once spoke to my dad or mum over fone, my sister has become single minded over him and just thinks he is the oinly person for her,i hate to see my sister upset all the time,and my parents, i dont know wat to do:'( she has done istikara n has good dreams but the mufty said dreams can jus b dreams sometimes and could not be the answer you are looking for.
please give me some advice on this my sister just seems mad over this guy and depressed when she dont hear from him and when my parents say you cant marry him. we are struggling as a family just for my sister to keep her happy and we dont want to lose her in anyway just over this guy.
we have shown her pious guys but she would say to my mum that she already has found a guy and is not interested in looking at anyone else. her friend in my street had argued with her family jus over a guy who is of different cast they did not agree but the girl still got married,and i have a cousin sis where here dad didnt agree but still she went pakistan and got married,she has bee seeking advice from my cousin and her friend,we dont want to lose her,we are 1 loving family and when things go wrong we always help each other. i know that if you marry someone and your parents dont agree this means your marriage is not blessed,my sis keeps trying out evrything and asking her friends.
i can go on about this all day but don't want to bore you all and as it is jummah today i must go and pray, i will stop here. but please brothers and sisters reply back as i don't know wat to do now:'(
jazakallahkhair x