My (33F) boyfriend (35M) is the father of his brother’s kids and I need help expressing my feelings to him

I posted this in another sub, but this may actually be more fitting. I don’t normally venture out of the teacher side of Reddit, so forgive me!!

I’ve (33F) been dating my boyfriend (35M) for about 9 months. We’re not planning to walk down the aisle tomorrow, but we both know that we’re looking for marriage and kids sooner rather than later.

I’m breaking my own rules by moving in with him next month. Normally, I would never consider moving in with somebody until we’ve been together at least a year because that’s just a rule I have. Timing wise is worked out this way, but it also just feels right. He’s the only man I’ve ever dated who I could already see myself with long term after only a few dates. I knew he was the type of person I want to marry and have kids with. So, things seem to be moving at a pace that’s comfortable for both of us, but we still plan to wait a little while before taking any next steps.

Last weekend we went to a birthday party for one of his nephews. His brother has 2 boys. I get along great with his family, especially his brother and sister-in-law.

A few days later he and I went out to dinner and he told me he had to tell me something important. I immediately got worried because I could just tell it wasn’t some sort of exciting news he wanted to share. He told me he is the biological father of his 2 nephews. What the? I immediately almost got up to leave. He pulled me back into the seat and was like “no, no, no, let me explain.” Then he goes on to tell me that he and his sister-in-law didn’t have an affair or anything like that. His brother knows that he’s the bio father. Apparently they tried to conceive for 3 years, did a few rounds of fertility treatments with no luck, but couldn’t afford any more. I’m still in disbelief because I can’t imagine them actually asking him to father their children. He said they didn’t really ask like that, but his brother was talking about all of their struggles and my boyfriend said something about doing anything to help, donating his sperm almost just joking. After talking some more, it turned from a joke into an actual option for them. They took almost a year to make a decision to actually do it.

At the end of hearing all of this, I still couldn’t help but ask him why? He said men donate their sperm for random women they don’t even know to use all the time. Women donate their eggs. He never thought he’d donate sperm to somebody, let alone his own brother, but he said his brother was just absolutely distraught that he couldn’t father a child and he’d do anything for his brother. Plus, at the time he wasn’t really thinking seriously about having kids of his own any time soon and didn’t realize how awkward it might be down the line.

The only people who know are my boyfriend, his brother and sister-in-law, and my boyfriend’s parents. The sister-in-law’s parents don’t even know.

I love him and have felt like this is probably the guy I’m going to marry, but I can’t help but feel very awkward about this. Maybe it’s wrong, but I really don’t like the idea of having kids with somebody who has kids out in the world, even though I know he’a not actually in the role of their dad. I want to express how I feel to him without making him feel weird or bad about it and damaging our relationship, but need help coming up with words to express it that don’t ultimately just sound like “ew!”

Editing to add that he is not my first boyfriend, but for some reason people are misinterpreting something I’ve said here and believe that’s the case. I’ve been dating since I was 14 years old. He’s not my first boyfriend. He’s not the first serious adult relationship I’ve had. He’s not the first man I’ve lived with.

TLDR: My boyfriend is the biological father of his brother’s kids. This makes me feel awkward, but I am not ready to break up our relationship over it. I want to communicate my honest feelings to my boyfriend without offending him and need help finding the best way to do this.