If you act like a jerk or treat her badly, your friend will not approve, and you might lose your chance of becoming close to the sister as well as your relationship with your friend.
This is one of the biggest dangers that you risk in dating the sister of one of your friends.
For instance, you might say, "Since we went to that concert with your sister Sarah and her friends in July, I've been having some pretty serious feelings for Sarah.
Keep your lips sealed tight.
Get to know her better through family conversations.
Now, just ask her out, like you would any other woman.
They can get a bit sticky at times, depending on the maturity of those involved.
Some friends have an invisible "friendship demarcation line" that they don't allow others to cross.
If that's your boy, don't do it.