So someone commented on my last blog about boundary stomping SIL "is she trying to be the childrens new stepmom?" I'm pretty sure it was jokingly, but that could legitimately be what is going on.
SIL and FIL are both dumber than average, but they are experts on everything. Example: SILs new boyfriend asked DH what he does for work. When he started to tell him about his job (in management), SIL interrupted him and started going on and on about what it takes to be a good manager and giving DH advice on it. SIL has very little work experience at all, and zero in management or in DHs field.
I noticed awhile back that SIL had started posting things about being a stepmom and what it means to be a good stepmom and so on. Also started referring to herself as a mom of 3 (including bfs daughter). Which I found odd, because shes been with her boyfriend less than a year and he only sees his daughter once a month because she lives far away. So she barely even knows the kid.
So actually it's not that far out there of an idea that she could have decided she is the ultimate stepmom and is going to be one to SD too. Would make sense why she asked to have SD one weekend a month from BM (when BFs daughter is up here). All I can say is good luck to her, because SD doesnt seem all that enthused about going to SILs on her moms weekends. And I know how it worked out for me trying to be inclusive of SD with the other kids.