I wouldn't feel obligated to invite her just because she is my FMIL. I am on ok terms with my FMIL but she is certifiably nuts. For example, one day she is ok with me, then 20 minutes later she is saying four letter words. I know this and part of the reason I did a DW was so she wouldn't be able to make it. I honestly believe if she was at my wedding she would ruin it. I don't if on purpose or not but I don't want our special day to be ruined by a person who was never a real mother to her son. My fiance is fine with it all since he knows how his "mother" is. A couple of years ago at his cousins wedding, she was so drunk she started to lick the ice sculpture.