being a milf is not about having children, it's about the social role as an unattainable romantic interest. mom's friend, teacher at school, your aunt, and ultimately your mom. a 16yo teen can be a mother but she's not a milf. she's a classmate.
20 yo mom, big no.
40 yo soccer mom or a teacher, fuck yes.
for me the core of the fantasy is the reversal of power relation, seeking to conquer the unattainable strict mom. your friend's hot soccer mom who was totally oblivious to you, there was no world where she would ever see you sexually. very closely related to a strict teacher-student type of relation.
that's why it doesn't work with a young girl who could just as well be your girlfriend anyway. a young mother lacks the social position to be unattainable, she's basically just your peer who had a child. easily in your league. the expected pairing.
same with siblings, unless the sister is significantly older. not just in years, but in status as a well. your age difference might be only a couple of years, but you're a boy and she's an adult woman living adult life. (I'm not into shota though, the male mc needs to be old enough to be fully responsible for his actions. old enough to make mom getting interested plausible.)
dad-daughter or younger sister doesn't work for me at all, because the power structure is completely opposite of what I like. a young girl is the easiest possible conquest. naive, stupid and easily manipulated into sex. the whole power relation of it simply disgusts me, it feels just intrinsically abusive because it's the one with power pursuing the powerless one. the easy-mode of incest.
if it were the other way, the daughter chasing the dad, I think that could work for me as it's the powerless party who's calling all the shots. but I don't play female protagonist so it's a no go.
an older woman can be totally out of your league, and not just because 'incest is wrong' but because she really is out of your league in all her social roles. she's the highest mountain, the unattainable peak, but somehow you're gonna find a way to worm your way in and get her on her back. and that's what I'm after.
this also means she should be reluctant to the idea to begin with, even if she somewhere deep inside wants it. it just ruins everything if they're throwing themselves at your dick the second they see it. it must be another uphill battle of breaking their mental and social defenses one by one or it means nothing to me.