What I'm about to ask is weird as hell but I need it.

I think there will be no shoujo with that concept. More like josei. Some josei have incest like brother & sister. But never found mom & son. And I don't think I can read it

Since we've been reading father x son then it will be the same if we read mother x son.

No I don't read father x son. Brothers are 'okay'.
I think father x son that I read are only not equal & ogawa chise, that was, I totally didn't expect it to be father x son lol.
But I think that theme will definitely go to josei

As far as I remember, I only stumble upon step fathers/mother, so not actually related.

But for me, Usagi Drop is the very definition of disturbing incest. Although they're not related, the guy has been raising the girl since she was, idk, 5? It's kinda....no...just no.

wait ._. u telling me they had that kind of relationship in Usagi Drop Σ( ￣□￣||)
I watched the anime a long time ago, i always thought it was just seinen (⊙…⊙ )

haven't found that kind of manga. but if you do please tell me. im curious too yknow since some of mangas always do the forbidden love between siblings. lol. ╮(￣▽￣)╭

......I'm very sory to have spoiled the manga for you. But. Like. It's such a heart warming manga righht???? You didnt expect that to happen right??!??! Neither did I. It's just so out of the blue...

It's ok, i didn't plan to read it, i didn't even know there was more (⊙…⊙ )
But. . .damn, it was one of the first non shonen anime i watched so i remember it well, it was so sweet . . . i dunno if i want to check out the manga (⊙…⊙ )

I recommend you not to. Let your memory be untainted...