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This is not a "love" or "romance" story.
There isn't really a ML/FL dynamic. It's just the MC being the caretaker of the ML's first love's kids and the two of them co-parenting.
The original Marchen was such a pitiful and tragic character. The MC will divorce the Ainz when the children are older because and she sees her job as being complete. They adopt the twins but neither have any children of their own. The MC will die shortly after the divorce.
The MC and Ainz don't "love" each anything more than friends.
Even when Ainz is an old man he does feel regretful but not in a romantic way but that he should have treated his "friend" (Marchen) better and misses her company.
It's funny the twins being the souls of the MC's dead siblings is the most plausible explanation the author could think of as a reason to why the MC would agree to raise those kids, given the circumstances.