I'm going to talk to a guy who asked out my sister
Are you attempting to get with the dude who asked your sister out?
Yea, I think he is going to deny her.
Your sister will probably be upset if you try to get with the guy that was trying to get with her.  You might want to ask your sister if she if fine with it first or it could cause a strain in your relationship that would have negative long term consequences.
It's not like she will care. I'm going ahead with it.
You will really loose your sister's trust if you attempt to get with the guy that you know she likes. Not only will this hurt her feelings, but this will also cause he to become pretty mad at you.