My Birthday is in two months and I plan to have a small party with some friends. One of the guests is a friend of mine. Problem is, I have a little (gay) crush on her sister, but never really talked to her except for at another Birthday party some time ago.
I’m not looking for a relationship and she’s probably straight anyway, but I would really like to be friends with her. Should I casually ask my friend to ask her sister if she wants to come too? Like “Hey, you can ask you sister if she wants to come too” or something like that. What should I do?
LemonVibe 
Yeah, just say “You can bring your siblings if you want.”
I don’t think there is anything wrong in telling your friend that her sister is invited as well. Just go for it you never know what could happen
DEFINITELY! What’s the harm of getting to know someone? Maybe she’ll take an interest in you.