Hi, I'd like to help you with your question.
It sounds like that the abuse the siblings suffered as children, which sounds particularly bad, has caused them to band together in a dysfunctional way.
Oftentimes, when children are abused, the way they conduct their relationships becomes dysfunctional as a way to survive and because it is the only way they understand. In your fiance's case, the fact that he was the youngest and the only boy may have triggered his sister, especially the one closest in age to him, to protect him. They may have also felt responsible for him because they have felt since they are older, they should have been able to protect him. The feelings of guilt, hurt and pain may have motivated them all to become a unit and protect themselves from all "outsiders". They look out for each other, but in a very dependent and enmeshed way.
The curious part of all this is why his mother is contributing to the situation. She is the other parent and since she may not have been able to stop the abuse, she should at least be trying to help the children overcome what they experienced. Instead, she is participating in it and keeping the children in a dysfunctional relationship. She may be doing this because she was abused herself, either as a child (likely) or while she was with the father. She also learned dysfunctional ways of interacting and may have taught the children to do what they are doing.
The obsessive relationship between your fiance and his sister should not be occurring after all these years. You are right, your fiance should be focused on you and his family should be second. The fact that he not only focuses on his sisters and mother but he allows them to treat you any way they wish then defends them over you says he does not value your relationship as much as he does his family. This is a dangerous sign and means that your relationship will be problematic unless this issue can be addressed before you marry.
Talk with your fiance about seeing a therapist. Hopefully, he has had the opportunity in the past to see a therapist regarding the abuse he suffered. But even if he has not, he still needs to address not only his abuse issues and the relationship he has with his family, but he also needs to address how he treats you. This is vital for your marriage to be successful. In any marriage, it is important that the partners put each other first and he is giving you signs that he has no intention of doing this in your marriage.
To find a therapist, you can talk with your doctor for a referral. Or, if you attend church, your pastor can help. You can also search on line at http://www.itsgoodtotalk.org.uk/therapists/ or http://www.bacp.co.uk/.
Here are resources that can help you learn more about how to address this issue:
Adult Children Secrets of Dysfunctional Families: The Secrets of Dysfunctional Families by John C. Friel Ph.D. and Linda D. Friel M.A.
Toxic Parents: Overcoming Their Hurtful Legacy and Reclaiming Your Life by Susan Forward and Craig Buck
Things I Wish I'd Known Before We Got Married by Gary Chapman
You can find these books on Amazon.com or your local library may have them for you.
Let me know if you have more questions or want further clarification.
I hope this has helped you,