Father x SonUncle x NephewCousins etc.

Father x Son // partially shota // Open ending - Hot & sexy incestuous fantasy...will it come reality? I think it will.

When Tomobe Yoshihito's cousin commits suicide, the carefree writer takes in her young, bratty son who's obsessed with buying his mother a grave. Soon their clashing personalities drive the child away, but he returns ten years later, determined to recover the money his mother left behind by any means necessary.

A relative of his mother

Son x his mother's lover

Uncle x Nephew (not sure if they are really related by blood)

Cousins

Cousins - Chapter 1

Guardian Relationship - Chapters 3 & 7
//Age Gap: 13 years// - When he was 21, a cafè owner took in a 8 y.o kid who had lost his parents in an accident. Now, ten years later, that kid has grown up...

Note: Bad quality scanlation

Ova (2) - Based on the BL novel Papa to Kiss in the Dark https://www.mangaupdates.com/series.html?id=39209

http://kissanime.ru/Anime/Papa-to-Kiss-in-the-Dark

Chapter 4 - Cousins