My fiance wants to move his addict mother in with us. How would you deal with this?

I really need some advice. My fiance and I have been together for 10 years. His family is super dysfunctional. They fight all the time, call each other names, his mother is an ex-meth addict who relapsed just a few weeks ago, and his grandmother is just a complete bat and I do not allow my daughter around her due to a previous altercation where she spit in my face and told me what a horrible person I am.

Anyways, over the last 10 years- no matter where we live, he has always offered his mother to stay with us and it ALWAYS blows up in our face. She's stolen from me, she goes through my things, she's fought with me, she's used in my home and lied about it...

She's pretty much burned every bridge with every friend and relative she has and we're now moving again (into a 2 bedroom duplex with our two kids and two dogs) and he had the nerve to tell her she can stay with us again while she "gets on her feet". She's always "getting on her feet" and we always help and it never goes well. I told him I was not comfortable whatsoever with her living with us and it's done nothing but cause huge fights.

He tells me that he's not going to choose between me and his mother, that he will always help her, that I just need to deal with it and not be a horrible selfish person. He's so blind by his love for her and their family issues and I refuse to raise my children in such dysfunction. She's not a safe person. Sure, she can be helpful and nice at times, but I do not want to live with her AGAIN.

Would I be in the wrong to tell him if he moves her in with us, I will not be moving to our new place and I would find somewhere else to go with our daughter so that he can live with his mom? I've expressed my concerns for years and nothing ever changes and I am sick and tired of putting my life on hold and helping someone who doesn't actually want to change or get help.

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I'm so defeated right now and straight up angry at my fiance f.or putting me in this position again knowing full well how I would feel about it. AND she would be bringing her cat and I am horribly allergic. He told me "they make allergy medicine for a reason".

I just need some nonbiased advice before I make a decision and walk away. THANK YOU.