The only evidence is circumstantial. Lev and the guest are obviously close because of the way we have all observed them with each other ... if you've failed to observe this by now I can't help you. They play in a way that some people as close as siblings do. Zoya seems very comfortable with them doing this. It would be somewhat unusual to allow her boyfriend to behave in this manner with a good looking female friend - not impossible but unlikely. From this evidence, many of us draw the conclusion that it is likely she is Lev's sister. Some of you do not share that conclusion.
BTW I have seen absolutely no evidence of incest. People who have are either just fantasizing, being naughty, or are predisposed to condemn others just because they exhibit behaviour outside their narrow experience. The 2 are just close, that is all. Nothing sexual or 'dirty' is going on here.