Oddly enough, no! I thought this too a little while ago given the style similarities and same name but she’s not related according to an interview:
If there was a specific moment I would attribute to being where I am today, it would be the moment I decided to take a mentorship with Rebecca Guay (no relation). At the time I was feeling very directionless. I knew I wanted to continue with my art, but I hadn’t found the place where my art belonged, I hadn’t developed my skills, and I knew next to nothing about the business side of illustration. The experience helped me find my voice, and it introduced me to the F/SF illustration community, which has been a huge source of support for me.