I know the feeling to an extent. Only I am the the bio parent jealous of the other one. I am noticing I am starting to develop a bit of unhealthy resentment to bio dad. He disappears for months then swoops in and is the "fun" parent for four hours and takes him out to chucky cheese and game stop. Then our son doesn't hear from him for 3 days even though his dad went months not even speaking to him. Really?? Yet here I am limiting in take of shitty food, unnecessary games, home schooling him, paying medical bills etc. It sucks.
I think it's okay to feel this way as long as steps are taken toward not feeling that way and you don't let your feelings interfere with their relationship. Which is my intention in my situation