I am not sure what my D16's options are legally when it comes to child care schedule, but I would think as parents we would allow her to be where she wants and not force her into the week on/week off schedule. I am going to visit my IC (whom I haven't seen in a few months) to talk about this from a non-legal standpoint.
As far as S18, there really isn't any room for him at W's place anyway. It is townhome-type detatched home with 3 small bedrooms. S12 and S11 will likely share a room and she will set up a room for D16 so no room for S18 anyway. He will be attending community college here in town next fall anyway, so he will be at home for a couple more years.
ME/XW:47 S21, D19, S15, S14 M:21 T:26 W moved 6/10 I filed 7/10 D final 4/12 remarried 8/12 W wants to R 12/10 and 4/11 but I decline