TTC while divorcing

My (still) husbands family thinks we’re totally crazy so I want to hear other’s opinions.

We split up 2 years ago, because he had a long affair and I found out. We are still good friends, we work together, and we co-parent great. There’s no way in hell we’re going to get back together, because the trust from my side is gone, and he is 100% aware of this. I raised my bonus son like my own for all these years (he’s out of the house now), and we have a young child together. Since we started dating we agreed to have 2 children together, and I’ve had a very rough time accepting that I won’t have more kids after we broke up. I’m in my mid thirties so I don’t have “too much time left”. The other day we had a conversation and I told him I’m saving to do IVF on my own, because I have a very deep desire to have another child, but I’m not ready to date, and I don’t want to speed date just to find a guy suitable to be a father.

He said that he knows how much I want another child, what I great mom I am, and that he wishes he could be the father of this child even if we’re not together. So we talked about it more and agreed this is what we’re going to do. We’re financially well off, we already signed an agreement regarding the divorce and our child, and he’s more than willing to add another child on it.

I’m longing for another child, and if both my kids share the same father, whom is a great dad, I feel it’s like winning the lotto.

But when I told my SIL’s they said we’re crazy and it’s irresponsible to bring a child into a broken home. (They were however supporting my sole IVF journey 🤔). So what’s you’re opinion?