There's this manga I read long ago, but, i don't know what it's title is. It's plot is, there is a boy that i think is waiting ,then, i think it's a grandmother of his who came and, informed him that his cousin will come. when he(cousin) did, he then asked his for his present. then, they got into a wooden bath spring where there he said that this cousin of his went away because of a particular conflict... then they become curious about their private Parts where the boy is amazed with his cousin's. then it then came when they are already in bed, the boy thought about his present so he went to his cousins room and there he got his present...you know that thing*... 😃
i think it's a shota but the seme is i think 16... and i think it's only a oneshot ....

Could be takaramono by riyu yamakami not positive though

Moved to Yaoi/Yuri~