On again/off again causing a rift between brothers

My BIL (35) was with his gf (35) for 11 years and before he proposed they had a rift and he moved out of her apartment for 2 months. After that rough patch, they got together, renovated, and he proposed soon after. She started continuing education in college and assuming she met someone because she went off on her fiancé and eventually told him she needs space and he needs to move out. It’s been a year since their break up and he’s starting to date her again, went on a weekend trip without telling anyone. I don’t care much about my BIL business but my husband is annoyed with it all, saying his brother has no integrity just crawling back to his ex, because he’s also lazy starting over again. He’s told his brother, if he were to get back together with her, he can’t take him seriously and can forget about future working partnership, because she will be putting her nose in their business. She’s quite unpleasant honestly, she never wants to be in family functions whereas for my BIL that’s a big deal.

My husband has said, he’s never liked her but tolerated her for the sake of his brother but now his brother’s immature behavior is putting him off. In a way, I understand but then again not sure why he’s so invested in it. I’ve told him, he doesn’t have to be working partners with his brother but at least respect his decision regardless. Is this odd of how he strongly feels about this?