I think I would move the office to the basement. I know you said there was no place to have a private phone conversation but it the phone use is limited, you could always have a conversation in the bedroom. (Of course, this doesn't work if the phone is used often and you need the things in the office (computer, files, etc) while on the phone).
If you moved the office in the basement, I would move your bedroom into the bigger room and the girls could continue to share the room they shared with you.
As far as boys and girls sharing a room, I would avoid it if you can. I know there are lots of people who share sleeping spaces but in western society, there is more of a stigma attached to same genders sharing rooms at older ages. Plus, I do think it makes it easier for teens who want privacy.
If you moved the office in the basement, I would move your bedroom into the bigger room and the girls could continue to share the room they shared with you.
As far as boys and girls sharing a room, I would avoid it if you can. I know there are lots of people who share sleeping spaces but in western society, there is more of a stigma attached to same genders sharing rooms at older ages. Plus, I do think it makes it easier for teens who want privacy.
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