My cousins dad was never around for her when she was younger that we know of. Well her mom got into a car accident and passed away now her daughter lives with her dad and she never sees her little brother. Her little brother is calling his aunt and unkle mom and dad because he don't really understand. Is it really right for her dad to get her even if that is his real daughter if he never was around before?

If he's a good father.. How old is she? Who does she want to live with?

As far as separating the children, I don't agree with that.

Who placed them where they are?
What you are asking is common.

I think that it is okay, if not great that the father has agreed to step up and take care of his daughter. It's unfortunate that he waited until this situation to be in her life.
It's typically too much to ask to require the father of one of the children to take on the other child, but it would be beneficial for both children to be kept together to at least have each other during this tragic time. Hopefully they can find away to keep these children close if not being raised together long term.
I'm sorry for the loss.

She was only 5 years old when this happen and she never knew herdad and her mom never wanted them separated but as soon as something happen to her it happen and there was really nothing no one could do