Why did either of you believe she was directly related at all? She appears much more like a governess, nanny, or some other variety of highly-trusted, extremely high ranking servant/teacher to the child.
Either way, the much more concerning spot of that scene is the emblem on her leg. Haruto's inner eye has one like that, but we've seen it on the members of the evil council too (I believe it is on the evil commander's neck, can't remember the episode though).
That friend is also more than happy to let him ravish her whenever he needs it. She seems to already regret pushing away his proposal, though it was good that she did so because it was out of guilt and not love (now she'll have to wait until he asks her again!). She's willing to have him take her to keep the rest of his friends safe. Rukino probably knows more about what he's going through and how he is suffering more than anyone else, she was mostly willing the first time, and is completely willing this time...
...and instead he choses the boys love method.
She probably shouldn't have chosen the words she did, he might have gone for it.