Life Hacks/People's general reaction to young Lexi
People who knew Xander before he age-regressed and gender-changed himself into Lexi will think of her as unusually grown-up for a child her age and generally have a raised opinion about her. However, they will not be surprised if he acts in a childish manner appropriate to his reduced age. Due to societal the complicated societal rules and sexual politics that are different for boys and girls and especially sensitive to age, due to Xander's and Charity's past history in the sexual department there may be some rather extreme reactions to a young Lexi that did not exist with young Xander.
Note that while opinions about Lexi will be different from what they were for Xander, relationship scores will not move immediately. The individual must interact with the age-reduced and gender changed Lexi at least once for the "program" to catch up.
Individual people
Brittany will remember that you were the only one that Charity did not go after back then, seeing you as either the one that got lucky or suspect that Charity might have tried to get you roped in but because you were such an advanced and smart child you refused to be involved.
Her reaction to you and March will be a lot more complicated. She does not know for sure whether or not you two are doing anything, although it certainly is incredibly suspicious for a 14-year-old boy to keep seeing a 10-year-old girl. However, her hidden desires from when you were Xander for the two of you to be secret gay boyfriends in a sexual relationship will be re-interpreted by the system and turn Brittany into a closet pedophile with voyeuristic urges to see boys and girls younger than her having sex. As such, if she suspects such a thing is going on, she will be less likely to obstruct it and more likely to try and see it if/when it happens, especially so if it involves Lexi who will become the main focus of her twisted pedophilic desires to see some boy with his penis inside you.
Charity will be somewhat aware of both the activities between March and the recently transformed Lexi in the context of Lexi having alwasy been a girl of an appropriate age, and her previous sexual desire for you as her brother plus the fact that she never touched you sexually will cause her feelings to move violently in the direction of wanting to protect you.
Charity will not like the idea of you and March being together, thinking it is inappropriate, and she will have no idea why you share a bedroom with Bastian but if she sees even the slightest thing out of line she will probably want to immediately demand Lexi move to sleep with her in her room, but could accept a re-arrangement of rooms that would have Lexi and Angel sleeping together as well beings Angel is your twin now.
Bastian is at the age where he wants to know more about his body and this new world of sex that has opened up to him since he has started puberty and begun having wet-dreams. He is a bundle of hormones and looks back fondly at the time when Charity touched his privates all over and let him see and touch the privates of his sisters, regretting that his mother caught them and now it's over.
He knows that, as Xander, you and March would do sex and masturbation related things when you kicked him out of your room. As you are now younger and female, he will re-interpret this as March doing the same things with you as Charity did with him, and he will want in on it even more desperately than he wanted in on it before, mostly for the chance to touch your privates.
Memories of his past interactions with you, especially your mutual masturbation sessions, will become REALLY complicated now that his brain has to re-contextualize them in the light of you now being a 10-year-old girl. He will recall that you were not just willing, but excitedly initiated most of your masturbation sessions, so he will not feel like he forced you into anything.
He will have a sense that what you have been doing is wrong, but due to the fact that you are a girl now he will also feel a large hormones-driven attraction spurred on by his familiarity with you. Despite knowing that it's as wrong as it gets, he will begin feeling stirrings of a pedophillic love and lust for you in his confused teen-age brain.