Yeah I agree with the sister complex, but also I think its because of the mentality that the only person who you ever hang out with, played with, discuss secrets upon (basically both a best friend and a brother) would be slowly spending less time with you as he gained has a lover, and that would result something akin to losing a part of you abit. And thus, the need to broaden your social group and time, to spend with others as to not seemingly cling or annoy her brother much.
Its like a situation where you spend less time with your bestfriend cuz they now have a significant other and you really dont want to bother them much as you want then to be happy with them and asking then out to hang out feels like youre disturbing their time together-- kind of sense
truly enjoyed hanging out with since childhood*
Yes I totally agree!(▰˘◡˘▰)
About his sister I was wondering about her feeling towards him too but I searched it up and it seems like she only like him as a brother but with everyone telling her she have a brother complex in her childhood she act distant and watches her actions with him so I think that the love she built up is just her taking out her feelings of love she couldn’t express as his sister .