I'm extremely attracted to my best friend's older sister
What do you mean by extremely attracted?
I'm extremely attracted to her, I'm gonna marry her
You may be making things complicated if you choose to become involved with a close friend's family member. This could put your friendship into jeopardy depending on how your relationship plays out. Just be smart if you decide to pursue her.
I decided to ask her out, she said yes!
Did you run it by your best friend first?  If you haven't yet, you definitely should before things get any more serious.  You still have a chance to do the right thing!