Kelly is too young to have kids
Wrong.
She's a doctor who used to work at the town's hospital. That's years of being a medical student, and however long practising as a doctor.
Add to that, the undoubted
years spent making porn (Adrianna mentions her popularity), and she's more than old enough to have had kids. Julia is old enough to have kids.
Danielle is the only one out of her and her sister who seemed to have little to no problem with having sex with Jason at all. She only had
mildly more resistance to the idea than Julia, who only needed to know that she and Jason aren't biologically related.
Rebecca has only ever had sex with her husband, and so far there's been no mention of kids.
Given that she's a teacher married to a rich husband, and one of the younger women Jason can fuck, I doubt she has had any.
Rachel's body wasn't "ruined" by pregnancies.
One pregnancy can be enough to leave a woman's body in a state that some would consider "ruined", but it's mainly down to post-pregnancy issues that have the biggest impact on a woman's body (hormones, diet, free time to exercise etc). As it is, her body was in pretty normal shape for a woman who's had at least 1 child. It's fair to assume her body was in better shape prior to Billy, as she wants to get back in shape and is clearly conscientious about her body shape.
None of this, in any way, is to be taken as indication that Danielle, Kelly, Rebecca or Rachel is Jason's biological mother.