That's... a huge stretch. Maya isn't even really Sage's sister because she wants nothing to do with the sorority. Sage is actively working to try to get Maya to come back, so whatever drama she may/may not have with the other sisters is clearly not stopping her from reaching out to Maya.
And I honestly, truly don't care who Sage assumes Chad's cheating with. What's that quote? When you assume, you make an ass out of you and me (or something). If Sage thinks it's one of them, she should ask them. If she's not willing to do that, then she should just get over it -- well, ugh. Not get over it... but like, move on from it/stop obsessing over it. If she can't ask the question, then she's not truly ready to hear the answer, so she should just stop trying to fin it. That's the mature way to approach things, at least.