Best friend's lesbian sister ⇐ Incest

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I have a best friend (let's call him Andrei), with whom we have been friends since kindergarten. and Andrei has a younger sister (let's call Ksyusha), whom I know from the cradle and who I raised on my knees. we lived in neighboring houses and my window is located directly on the balcony in Ksyusha's room. Our parents very often joked that I was raising a bride for myself and, in general, such classic jokes. and in childhood, she herself very often hugged me, kissed me, etc., calling me her husband. but our communication abruptly stopped when she turned 11. You know, transitional age, all that. at 12, she even told her mother that she was a lesbian. Aunt Ira came to my mother and complained.
and I knew this, because I watched from my window more than once how Ksyusha was suckling with her classmate. And then I entered another city and did not see her for about 3.5 years, but I regularly talked with her brother and he sent me photos where she was already fully formed and just very hot.
Almost 2 years ago, when she just turned 16, my parents arranged a christening for my sister, to which Andrei's family was invited. But Ksyusha didn't show up because she wasn't feeling well, so I was asked to take her a cake, although in the end I carried two bags full of food for one miniature Ksyusha.
When I went to them, she sat in the kitchen crying and ate.
Then we began to get closer.
Ksyusha shared with me that day her life and told me why she was crying. it turned out that she was abandoned by a girl with whom they had been together for almost 2 years. I calmed her down, fed her with everything that was handed over for her and sincerely talked like that. From that moment we began to communicate as much as in childhood. Andrei, of course, noticed this and began to constantly throw in jokes, such as "do not knock up your sister", "I love you, but not to such an extent as to be related", etc. Naturally, he did not know how close we were, but as they say, there is some truth in every joke.
She was very comfortable with me, so after a while she began to hook herself. It all started with intimacy. First it was "oh, I accidentally", "oh, it was for a friend", and then "I'm better in white panties or black ones." And of course we started having sex with her. At first it was awkward, all the same, the sister of a best friend, plus I knew her from childhood, but she was very assertive. So we sleep with her until now, but yesterday Andrey came to me. we occasionally drop by one of the two of us and play shooting games all night while drinking beer. And apparently Andrei went overboard, because he began to talk about what he was fucking Ksenia.
And he began to do it a very long time ago. I won’t mention her age, but he said that he began to pester her back then - he just touched her. In order to sleep with her, he came only when she was 12, but he rarely did this. Now I do it on a regular basis.
We are not officially dating Ksyusha, so I'm interested to ask what to do? just to score and not lose the option of regular hookup and suction? Tell Andrey about your relationship with Ksyusha? talk to Ksyusha about it?
By the way, I have a theory that she switched to girls because of her brother. what do you think? Do you have any stories of similar situations?