untrustworthy acts (lying, cheating, stealing, withholding important information)
physical/mental abuse to me or the kids (zero tolerance for either)
substance abuse
I'm not worried about the above three in the least. We've been married for 20 years this year, together for 28.
But the next one.....complete dealbreaker:
His mother coming to live with us. Not even joking. She is transient and unable/unwilling to support herself. She has 8 children. If it came down to is and NONE of them would be willing to take her in, and she came to live with us, DH and I both know that it would be the end of our marriage. We have talked about it before. He would be willing to still take her knowing I would move out. Not that he wants either to happen, and I would never make DH let her live on the street before moving her into my house, but there is -zero % chance that I would stay and live with her. She has made the rounds of all her kids and we are one of the only two kids she hasn't lived with yet. In her words "H*ll NO!" would she live with me, and she feels pretty much the same about my SIL. We concur.
*Before flaming with the be nice to your MIL comments, she has been physically and mentally abusive to my kids in the past and will be again if given the chance, as she has also been with many of her other grandchildren, and is verbally abusive to me when DH isn't around, and has stolen from my home multiple times. I refuse to be around her alone because when he is there, she is sweet and nice to him and ignores me, which is the way I want it and when DH is not there, she is passive-aggressive and nasty to me with her comments and innuendos about how terrible of a person, wife, and mother I am. We spend very limited time with her, the kids are NEVER far from me when she is around, and we changed our locks so she can't come in when we are not home and steal from us. And it's not old age/dementia/etc. She has been like this since I've known her - I was 14 and she was 37 when we met.