Nugget: I take it your fiance was not baptised and yet is being treated like a Df'd person.... If you MIL was a drunk or a drug addict you wouldn't think twice the threat is so clear.
Blonde: This is definitely a toxic relationship and dangerous. You should both be strong and not let this abuse continue.
Jaime Bowers: You don't have to be rude, but you must be firm.
ALL GREAT ADVICE! You don't have to be rude, but you don't have to allow you or your loved ones to be abused. Yet, your best approach is to always appear reasonable and understanding and willing to compromise.
Some thoughts on dealing with the situation:
I am assuming your fiance IS/WAS a baptised JW or the family would not be totally shunning him. (1 Cor 5:11 "But now I am writing YOU to quit mixing in company with anyone called a brother that is a fornicator.....")
If he is NOT baptised (as Nugget suggested), then I would ask your MIL what is her scriptural basis for shunning him? In JW legalese, he is simply a "man of the world" or a "weak" unbaptised publisher and should be encouraged, not shunned.
If he IS baptised, then get the following quote off of the WT-Library CD, from the article “Is it Wrong to Change Your Religion?” in the July 2009 Awake p29: “No one should be forced to worship in a way that he finds unacceptable or be made to choose between his beliefs and his family.”Is she trying to make him choose between his beliefs and his family?? Show her that is contrary to recent (2009) WT doctrine. (A quote from a non-witness website won't be acceptable!)
Also, get the quote from 09/15/81 Watchtower Pg 29, Par18: "The second situation that we need to consider is that involving a disfellowshiped or disassociated relative who is not in the immediate family circle or living at one’s home. Such a person is still related by blood or marriage, and so there may be some limited need to care for necessary family matters. Nonetheless, it is not as if he were living in the same home where contact and conversation could not be avoided." Determining what are "necessary family matters", i.e., seeing one's grandchildren, is a matter of individual conscience. While an elder or MS might be at risk of losing their position if it is determined that they abuse the loophole, there is no action that can be taken against his mother (unless she is a pioneer). She will be giving up the only opportunity the child has to be exposed to the good influence of a Witness environment.
A former Circuit Overseer, who was a friend of my parents, said that one of his life's greatest regrets was refusing to go to his son & DIL's wedding, which they had at a non-religious site by a Justice of the Peace (not a clergyman) as a compromise to the Witness family members. His regret was that it alienated this kind, reasonable girl so badly that for years she never agreed to listen to the Kingdom Message like she might have if they had been reasonable on the matter. He sites Jesus example of healing the woman with the flow of blood in Luke 8:43-48. In touching Jesus, this woman broke an absolute rule of the Mosaic Law. Yet, Jesus did not condemn her, but healed her. The WT uses this as an example of Jesus being "reasonable and yielding" vs. ridgidly enforcing the Law.
Finally, you might have some hope in convincing her that once you are married, he will no longer be a "fornicator". The scripture says to not mix with anyone who is a "fornicator". You might have some success in noting that to continue to do so, would be "adding to" the Word of God.