My half bro had a baby with his ex girlfriend. Now me and her like each other and are going out. Is that weird or...

Why would that be 'weird'?

Is it causing any issues in your family or with your half-brother? Do you feel as though she is using you to be close to him?

As long as there are no issues and he isn't ditching his responsibility to his child or she isn't using you to get something from him, then I hope the relationship works out for you.

Good luck.