The majority of women I've had sex with had steady boyfriends at the time, but were open because their primary guy was either far away, or they had very different sex-drives and I just got their overflow.
You might just want sex more than she does - a woman who wants a casual relationship doesn't necessarily want to fuck any more than a woman who wants something serious. She could sense that you want more sex, and since you're just casual (and that's all she wants) she has no problem trying to set you up. It sounds like she might just be acting like a good friend.
The whole sister thing, on the other hand, does sound really sketchy. A serious ex of mine dated a guy for a year, and then within 6 months of them breaking up the guy started dating her sister (which lasted 4 years.) That always seemed off to me; I never really liked her sister and I think that was probably the main reason why. If I knew that one of my brothers was with a woman I'd file her away as 'permanently off-limits'.
All that said, there's a reason why you hear a lot about cuckolding (the man only has sex with one woman, but that woman has sex with many men) as a 'fetish', and I think it's because a man sleeping with multiple women while being more serious with one is so common that it's not even considered a fetish any more. Every woman I've been serious with has confided to me at some point that hearing about me with other women turns them on, even though it's not something they actually want in the relationship.
I'm very serious with my girlfriend (we live together) and right now she's the only woman I want to have sex with (note: that doesn't mean I don't find other women attractive, but all things considered there's no one else I would be with.) When she thinks other women are flirting with me she seems a little bothered, but every time she's seen a woman who she thinks is hot getting friendly, my gf fucks like crazy when we get back home. She still sometimes sounds upset that I had another woman in my bed the very morning of the day I asked her to be my girlfriend, but every time she brings it up her hand's on my cock soon after.
So I think your girl is at least one of:
a) just being a good friend and trying to help you out
b) sensing your drive is greater than hers
c) turned on by the thought of you with other women
Any one of those things sounds like a good deal, so I say go for it -- but stay away from the sister and so-close-they're-like-sisters level friends.