How do i tell them, how do i begin to explain??
Asalaamu Alaikum, My sister you should first tell someone within your family who you feel most comfortable with and will help and support you through this. You must not hide these serious matters from your husband.
Obviously this is a very big thing which is going to be exposed to your family but they deserve to know and you should not just think about how hard it is going to be for you to come out with this but you should think to yourself
"My husband and sister have a right to know about this so i cannot hide it from them any longer".
Your husband has a right to know if another man is trying it on with his wife and your sister deserves to know what kind of a man she is married to and your family also deserves to know what kind of a son in law they have.
The longer you delay it the worst This situation will get. You should never want to hide such a thing as your family deserve to know the truth about him.
So firstly tell the person who you feel most comfortable with and they can help you expose it to the rest of the family or you can write a letter as the sister has stated whichever way you feel most comfortable. But you must not delay this any longer and you should never stay in a house with any non mahram again as it is not permissable to do so. You should not be ashamed going to your mothers house as this is far better than being alone in a house with a non mahram.
Ask of Allah to make things easy for you and for justice to be done inshallah.
And Allah knows best in all matters