WWTBC Witches and Demons Series
FOUND IT!! Had to go all the way back to books purchased in 2018.
Love and Magic Series by Nadine Mutas
The second book, To Win. Demon's Love was the one with the witch turning into a demon. It was a spell that was turning her.
Thank you to everyone that tried to help me find it!
Original post:
I remember this book was part of a connected series. In the first book I think the witch FMC summond a demon MMC? Which was highly frowned upon.
Set in modern era, just happens to have witches and what not.
The later book in the series that is driving me crazy has the FMC slowly turning into a demon, I think because she got some of the MMC's blood in her...or something? They are fated mates. She freaks out at one point when she finds out that although he has never been able to, er, break the seal and have a full orgasm, he has had a lot of practice trying before he met her.
He has a really lovely family, which turns her perception of demons on its head. He is a loving sibling and doting uncle, and she never considered demons could be like that.
Would have been available on Kindle Unlimited.
Hope you amazing detectives can help me out!