So my best friends (close friend) sister likes me and she really wants to fuck. So so direct. And she liked me for a while. But it's my friends sister. Should I go for it? And keep it a secret. If I keep this a secret, its a second secret of a friend I would fuck. What to do?!

Its too personal of a question, it depends on a lot of factors, ones I can think of are:

-Relative ages between you, your friend and his sister
-Your friend's Character
-Sister's Character
-How did your last "secret" turn out?
-Is the sister "Clingy"?
-Could she demand certain things from you to keep your friendship intact?

without many details in your questions you might end up getting very vague and undesirable answers not because of malice but because we might simply guess at things.

When in doubt remember "Bros before Hoes" take it with a grain of salt because sometimes your friend's sister might be a great woman who you can have a awesome mature relationship with, but if she just wants to get fucked, I'd say leave the task to someone else.

There are plenty of girls DTF out there and if for whatever reason you're in this community and your friend's sister wants you, you have good chances with the rest of the girls in your area, keep your chin up!

Best Luck!

It is almost certain that you will lose your friend's trust if you do this.

It really depends on your friend's personality though. I mentioned to my best friend that her sister is legal now and I would have sex with her. She pouted, but I know for sure that if I sleep with her sister, at worst she will be pissed at me for couple days. That's all.
But there are million pussies out there, I wouldn't pick a one night stand over a friendship that I invested a lot of time in.