My husband's mom and ex will be the end of us

I've posted about my MIL before but never combined it with the other bullshit that comes with his ex and how the two make my life hell. This story is a novel is I'll try to shorten it as much as I can. My husband has a low life ex who has 3 kids (only 1 with my husband). The 2 older kids are 16 and 12 and she recently gave them up to their dad which turned into them in foster care cuz he is a drug addict. The one she shares with my husband is the youngest and she only sees her every other weekend. She is not an addict she's just a pos who doesn't like taking responsibility for her kids. This ex has stolen money from my MIL and cheated on my husband a million times when they were together. For some reason though my MIL lets the ex hangout at her house like they are buddies and will defend her when she is being her usual pos self. I can't understand why because there is no reason to like or respect her after all she has done. She will be rude as hell to me but nice to the ex. Oh and this ex lied to my husband about his daughter really being his but she is 10 now so obviously in my husband's eyes it doesn't matter. It's just the fact that she lied about something that huge and doesn't even see her and makes us do everything. This would all be fine if my MIL saw all I do but no she bashes me constantly and kisses the exs ass. I'm at a point where I'm going to leave. My husband will defend me to his mom but will also take her side. He never just takes my side even when I've done nothing wrong. His parents live a neighborhood over and he calls them several times a day so they are constantly around. It's really hard to deal with all I do when my husband never fully takes my side. There are so many examples of ways the ex and mom have made my life hell but it would just take too long to write out. If this was your marriage what would you do? I honestly don't know what to do. My husband and I have a wonderful relationship in every way except for when it comes to all this.