YA Romance I read around 2015-2019 (possibly part of a series)

[Reader]

• Two sisters (one very pretty, the other the main POV and overweight).
• The overweight sister tries to lose a lot of weight and a guy helps her; they start to like each other.
• I specifically remember a scene where she goes to his house to swim and they end up making out.
• Near the end she gets news about her father — I think he might’ve been a sperm donor (or smth like that, not 100% sure).
• It was definitely published (not fanfic) and I’m pretty sure there’s a second book / sequel that came out around the time i finished reading.

Any title/author ring a bell? Thanks!

[Margaret]

Would it be Life in the Fat Lane by Cherie Bennett?

https://www.goodreads.com/book/show/7...

I don’t recall the sisters portion, but there are similar scenes and a devastating pool scenes that stands out in memory.