https://pikath619.fanbox.cc
https://www.pixiv.net/users/3314710

Title "The first lewd act on the beach of my daughter and boyfriend"
so daughter has boyfriend now, before she had sex with papa.

Base +8

@H0R0
If I understand correctly, "娘" can also mean a "girl"/"young, unmarried woman" "https://danbooru.donmai.us/posts/4666879" .
So, maybe there was no sex with dad
But i can't be sure

Base +10

If you follows this author on pixiv you can clearly see he has 2 female characters, this girl and 1 other looks the same but with longer front hair which created before this character, this one designated as "daughter" the other designated as "mama" even has "daughter" and "mama" in one work.

https://www.pixiv.net/en/artworks/93941482
ハロウィンの雌猫ちゃん親子♡
今度は親子で猫さん
Halloween female cat parent and child ♡
This time, parents and children are cats

https://www.pixiv.net/en/artworks/82171047 https://danbooru.donmai.us/posts/3947855
急に声をかけてきた清楚なJKと結婚した話 17年後のママと娘
親子でぺあるっく
A story about getting married to a neat JK who suddenly called out to me 17 years later, mom and daughter
Parent and child

author didn't give them name, just "mama" and "娘" which of course can be "daughter" or "young girl" but with how both are in same work and designated as "parent and child",

so yes, this can be "young girl and her boyfriend frist sex at beach" and "daughter and her boyfriend first sex at beach" at same time.

Base +8