Hey, i was in a similar situation but it wasn't my sister's friend. The BIGGEST threat to you is your sister SPAM. If she's on your side then this should be a breeze. If you're sister doesn't approve of what you're doing however, then you will almost indefinitely have to use an indirect approach to open.
I'm assuming that she's gonna be there for a few days (or enough time to run game) and your sister will always be with her. If this is the case then yeah I'd advise an indirect approach.
Opening:
As far as opening goes anything should suffice. Since she's gonna be around a lot then the conversation pretty much takes care of itself. Talk about great things to do, the places you've visited ( a good idea to screen her for qualities you like e.g. do you love to try new things, are you adventurous or just the typical coach potato girl etc.') There's not much to opening in this particular case just make sure you maintain the thread of the conversation. The hard part is that your sister and her are probably gonna be talking a lot and catching up so i can't really help you out when it comes to 'what to say.' As long as you don't bombard her with questions you're fine.
Isolating:
Isolating is probably going to be the hardest part. If your sister is there then you're going to somehow have to get rid of your sister or wait till she's alone with you. If she's gonna be there for a while then a good idea would be to game her while she's with your sister using light touching and slowly building up the kino. Once she's attracted enough then you can organize a time for just you and her to go out while she's there. If this is not possible then you'll just have to sus it out and wait for your sister to be pre-occupied with something else while you escalate.
Closing:
I'm guessing you want to f-close. This is gonna be pretty tricky, unless you do manage to take her out and then bounce her back to your place for one of the days. But like i said, if you're not able to organize a day for you and her to go out then you might aswell forgot about the f-closing. If you want to kiss close then you should make sure you have enough compliance. Make sure she agrees to simple stuff like a hug, pat on the back handshake and see if her body language is opened or close. From here provided you've got enough comfort built and you have at least 3 IOI's then you should go for the kiss close. There's many ways to do it but I normally play the 'trust test' game and while you hold her hands and tell her to stare in your eyes you begin triangle gazing. (look at her eyes then mouth and repeat.) Ask her how good of a kisser she's from 1-10. Once she replies go in for it... As for the number close, that's pretty easy, before she leaves grab it and say you've enjoyed her stay and would love to keep in touch.
I didn't run through a certain type of method, instead just told you about what you could do to open/isolate/close her. Otherwise i'd be writing out an entire guide.
Anyway goodluck!
CHAT