MIL constantly talks about how much she loves my husband's ex

My husband's ex is extremely involved in the family. Mainly because she's best friends with my husband's brother. Everyone thought she and his brother would end up together but his brother is gay. My husband dated her in college. My MIL brings her up every time we see her on how she wished she was her daughter in love and one of her son's married her. We were there over the weekend and she was there because her and his brother lived together. His mom asks his brother every time "Are you still gay?" His brother will roll his eyes and say yes and she will be like "Just checking!" His mom at the dinner table brought up how she had years ago read my husband's journal and he talked about loving having sex with her! My husband of course was embarrassed and asked why she was reading his journals to begin with and she said it was her house she can read anything in her house. His brother said this is why he never left anything personal here. He left that at their dad's. She said she just hopes I can satisfy him the way he claimed she was when he was 19! Everyone was embarrassed and she and my husband's brother ended up leaving. I don't understand why she does this. It just makes me hate his ex and I know it's not technically her fault she's considered family but to constantly be reminded he was with her and that his mom prefers her is irritating.