Girl. Let me give my perspective (and this is my armchair psychology take on things):
It made total sense she’s acting this way when you described his home life. Basiclally, your Future Mother-In-Law is freaking out because she just realized she’s about to “lose” her primary partner/support/fill-in SO in life. She probably doesn’t even realize, on a conscious level, that this is what’s bothering her.When people’s sense of security is threatened on such a deep level, it causes them to act our in horrific ways that make people around them say they are crazy, or being unreasonable, etc etc. She’s upset and doesn’t know why, so is coming up with ANYTHING SHE CAN to justify her actions. So what will happen is first she’ll be pissed off about the guest list, but then she’ll say oh no, I’m just pissed off because you’re ‘impure’, and then be pissed about the cost, then change it to something else, and around and around – because NONE of those things are actually what’s bothering her. She doesn’t know what’s bothering her, but she’s going to kick and scream to make sure this wedding doesn’t happen because that’s what she knows will make herself feel better.
DO NOT TAKE IT PERSONALLY. It’s not about you, it’s about her boundary-less relationship with him.
Your Fiance did ABSOLUTELY the right thing by standing up to her, because if he didn’t, her desire to never let him go would just grow stronger and stronger and remain unchecked forever. Setting a boundary and putting his foot down, while totally painful and awful in the moment, is absolutely the best thing to do in the long run.
My prediction is this will get worse before the wedding, then completely disappear, and she will show up and grovel and be there totally happy for him once she realizes she is losing this fight. DO NOT GIVE IN TO HER!!!! Go forward with the wedding and be happy, and be confident you two are a team and she will figure herself out eventually.
I haven’t read everyone’s responses, but yelling at her, calling her out, arguing with her etc. is likely to get you nowhere – because she’s not making a logical argument, so using logic with her WON’T WORK. Just try to say “we appreciate you’re upset about (x: moving target), but we are moving forward with this wedding and would love for you to be there”. Don’t engage her anymore than that and I’m sure eventually she will fall back in line, and probably have more respect for you at the end of the day. If not, definitely come up with a “team strategy” for how to handle her in the future. One good rule is you shouldn’t deal with her directly ever – let Fiance handle all drama with her and keep yourself out of it.
Good luck and please keep us posted.