So for a little backstory: My FI's mother has a lot of issues: she has been in jail on multiple occasions for very serious crimes. She was never a present mother and skipped from boyfriend to boyfriend all of her children's' lives. Fortunately, she taught her children how NOT to be/act and most of them wound up becoming successful and put-together people. Since my FI and I met, FMIL has been diagnosed with bipolar disorder, has been given medication, and has calmed down immensely. However, she is still homeless and drinks/does drugs. Because of her past (and her need for a roof over her head), her boyfriend-picking tendencies are not the best. Here is where it gets confusing: her current boyfriend is the uncle of my FI's nephews/her grandchildren - yes, Grandma is dating Uncle. To outline this further: My FI's sister (my FSIL) Ashley* had two children with Joe. Joe and Ashley broke up a few years ago. Joe's brother, Andrew, met FMIL during Ashley and Joe's relationship. A little over a year ago, FMIL and Andrew began dating (she is 51, he is 26). Andrew is not a nice person - he is very selfish and just an all-around bad egg. He cheats on FMIL, can't hold a job, and has taken advantage of every person he has ever come across. He has stolen from FI (and has taken money from my wallet, even). He is verbally abusive both to her and to everyone else in FI/FMIL's family.
FI has expressed that he does not want Andrew at our wedding, for many, many reasons. What do we do? FMIL has expressed on multiple occasions that she loves Andrew more than her children and that he comes first in everything. When FI told his mother how he was feeling, she said she would not come to the wedding if Andrew was not invited (I have a feeling she would end up coming. She's very manipulative, but FI is her favorite child (yeah, I know... great thing to say to your other children) so I know she would not miss his wedding.)
I'm totally at a loss for what to do. FI really does not want Andrew there (nor does anyone else in FI's family). Do we have to invite FMIL's boyfriend? Do we have any other options? Do these special circumstances warrant not inviting FMIL's SO, or are those rules non-negotiable? Any advice would be greatly appreciated.
*Names changed because, well, you never know who is reading these boards.