Why are my foster & biological mom grudging against each other & they both DEMAND credit they don't deserve?

They're probably holding a grudge against each other because you actually turned out better than the both of them. Now that you're grown and have a college degree, they probably both want to take credit for your good choices in life and reap the benefits that come out of it. I wouldn't worry about it too much. That's something between the two of them. Don't let their actions change who you are or how you think of them already.