I'm pretty sure it's been hinted that she inherited the previous Mai's romantic love, but isn't aware of it. Though it's been a while since I read that and maybe I misunderstood
By my impression, she seemed pretty aware of it. I thought it was more that she thought there was no chance of him feeling that way about her since they're siblings and he is known to not care about her that much or be interested in her in the original time line. Since her entire character revolves around her bro-con tendencies, she never had any issue with this and just wanted to help him avoid the death game ending.
There was an earlier conversation where she said she didn't think the kind of impulses he has are a problem as long as the person can find a less destructive outlet for them, so her thought process has been helping him do this. As a result, him actually being interested in her would be a situation she hadn't thought about how to handle at all, as it's categorized as a "definitely can never happen" thing, and therefore not a feeling she felt the need to act on directly. Or at least that's how I saw it - not denial of the feeling, but just denial that she would ever have a chance to act on it.
This will be a slightly boring status quo if it continues for too long, so I'm glad we're getting some sort of change here.