Introduce yourself to the brother and befriend him so he knows youre not sleazy douche before swiftly moving your attention on to the sister. The brother always knows guys will hit on his sister, he is just there for protection or perhaps he is sarging himself. A good wingman could occupy him in deep conversation or you could get a female friend to chat him up to distract him. The dude knows when his sister is DTF, all he is looking for is that sign from her (usually a facial signal) of "I like this guy, go away and leave us alone so he can seduce me!"