Mother In Law

I have no idea how to handle my future mother and law and his sister! We have a wedding planned in 8 months and by the time we are married we will have been dating over 2 years. I saw little problems with his mother being controlling and major problems with his sister. His sister has no life and never dates, and did i mention she will be 23 years old. He moved to be with me in a different state, and now I am changing him and he is not a good brother at all and she is losing him. Every change she gets she voices her opinion that I am not good for him and that she sees area in me that are for concern, she tries to persuade him to put his family first and when we divorce he wont have a family bc he is choosing me and cant balance. This has been going on for the whole relations. Now added on his mother is flipping out that we want to switch thanksgiving and xmas every year for our familes. She has been crying and for a week now she is using everything to get us to always spend time with her at xmas. She started by telling him he wasnt a good son and didnt love his family, then making him feel guilty and said his dad was sick, then started blaming me and my family....now she is insulting my family in which are relaxed and dont care what we do. I am unsure of marrying him bc i dont want to deal with his family like this for the rest of my life and now know she wont stop until she gets her way. It is stressful and making me sick. The hard part is him and i have a wonderful relationship, openly communicate and never fight. Please help with any advice.....i really need some kind of strategies for him or I. Thanks