A college student falls in his love with teenage babysitter only to find out he’s dating her cousin.

I think he must win her back by killing her boyfriend.

That isn't clear from the logline...

"When he falls in love with his childhood babysitter..."

He had a babysitter when he was a kid.

This logline isn't clear:

I took it to mean when he was a young teen he had a babysitter, now that he is older he has fallen in love with her. In other words, his teenage babysitter. The babysitter he had as a teen.

But this is written in such a way you may be right, maybe the babysitter is a teen and he has fallen in love with her.

MIght want to correct the typo - falls in love, not falls in his love.

Also who are you marketing to with a logline about a pedophile committing incest as a comedy?

What is stopping the lead character from breaking up with his current girlfriend, so he can be with the girl he loves. (I mean it should be both clear and in the logline)

Because the thing that is keeping the lead character from being with the woman he loves is the central conflict of the story.

The lead wants to be with his former babysitter (but this specific thing is standing in his way)