@StaticBat83Shamelessly planting my kinks in what seems very receptive idea.
The old guy lives in a country house with a sprawling property. Inactive farm, like, as I doubt he's very rich. Lives alone, as mom, his (step-)daughter with her daughter have moved to the city.
Perhaps he got same kind of a medical scare (nothing to require constant care, or maybe he's refusing it) but enough for mom to worry, or perhaps grandmother had died recently and mom fears how the old guy be out there alone, or perhaps there's some other reason in mom's life (perhaps she's in a new or troubled relationship the daughter is *incompatible* with at least temporary) or in our girl's own life (too blatant "too soon" interest in boys, or a certain bad boy, may qualify too) why she's "punished" with spending the summer with the grandpa. Or some combination, or something else entirely.
The place is very secluded. There may be neighbors, but not close, and the relatively close may be mostly other old people or otherwise very uninteresting for our girl. Although I wouldn't be against if she met a girlfriend or even a guy to tease, but never get quite close with (at least not until... the very end, after she's knocked up by grandpa already?)
Mom works, and visits irregularly and at times quite unpredictably (if she does have boyfriend, she never, or almost never brings him out there -- that relationship stays in the city).
When mom's home, they both skinny dip in the nearby river together, for a beautiful mom-daughter time. When not, the girl may be a bit reluctant to do so alone at first, but soon stops to care to wear a swimsuit (she may turn to the neighbor girl for courage and in the result introduce her too to the great delights of skinny dipping).
It's both mundane -- mom sort of always did that there -- and exciting since girl associate nudity to sex and she tries to catch the grandpa (and/or the neighbor boy if he's introduced) peeping -- what she suspects may happen, but if (either) does, with very respectful secrecy. Nevertheless, this may renew or even introduce her suspicions grandpa is also her dad).
She's bored out of her mind and starts to run around naked more and more for a thrill. At first trying to hide from grandpa, then, to deliberately tease him with "accidental" and controlled exposure, then eventually blatantly out in the open, switching to near full home nudism when it's clear he doesn't mind.
Well, apparently he does, to some extent, as the relationship between them do develop into sexual activity.
He may fuck her for the first time out there outdoors by the river. And she then learn that's how the two previous generations of women in her family had both been knocked up -- and always by "wrong" someone.
And grandmother perhaps right where they did it now, but not by grandfather, although he got to watch. She wasn't his girlfriend then, but he salvaged her afterwards that other dumped her for the shame of a kid out of wedlock. (And said neighbor boy (or girl) may have seen her getting it from her step-grandpa)
And on said camping trip where mom pulled the gangbang, she too went to secretly skinny dip, away from the rest of the group, and apparently in a "wrong" place and time, but that's how our girl come to be.
***
It may be worth to adjust mom or/and grandmother events so the two stories within story (as told by the grandpa) would rhyme even more between them and with the main character's outer experience forming a cycle (where the last may be the most deliberate, controlled and happy, at least within the scope).
That may also help to keep those (relatively) concise despite both being very exciting -- by referencing shared details.
It should be considered that grandpa may be unreliable narrator telling one or both of his stories about her mom and grandmother. I doubt he's outright lying that he's unrelated by blood to the girl, but doubt may be reintroduced by subtle contradictions.
