Lot of great advice, I think I'll just call the one who sent me a message, invite her out tonight. Ok, but should I say "does your sis want to come out as well", or just not mention her at all?
No. Insist on including both sisters in any activity. Don't ever advance the relationship with either of them, in conversation or physical closeness. Let each sister take the lead with you all the time. If they ever put you in a situation to choose, get tired and go home. Never talk to one without mentioning the other. Treat them equally. Time will sort this out naturally.