What mixed messages ?
It was clear from the starts that Nikki will do everything she can to put her brother in her bed. Not because she madly want to have sex with him, but because she want to teach him how to get the chicks and how to please them. That's what she did, nothing more, nothing less. She just included Molly in the process, because she was an even more desperate case than the MC.
It happen that she liked it, partly because she had no action on her side, and so she wanted more, as long as it was possible, but that's all. Like I said
one and half year ago, she's the corruption factor, not the main target. And I can add that Molly always been the said "main target", the other girls were mostly here as proof of the said "corruption".