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.
Response to the spoiler
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MC will what Σ(  ̄□ ̄||) omg... well at least she doesn't love him and will not stays with him like many other sadly generic stories. At least she will fulfil the need of taking care of her siblings ヾ(❀╹◡╹)ノ~
Anyone who comes between siblings is trash. Which is what Moa did.
She slept with Yura and strung him along and then sleeps with his brother only to get back with Yura.
Causing a rift between the brothers (even if they are "fine" in the end) that won't ever be repaired to what it was.
Moa isn't someone who deserves a "happy end" she is a selfish and feckless ho.
Both brothers deserved better. Especially Yura.
I honestly have no clue what's going to happen next.