Sex is not the critical factor for 兄弟情/姊妹情. It's all depends on the relationship and environment for their growth. Your effort to build it up is more important.
Starting from last Saturday, Kei Kei took something with her when she go school:
Sat - Doraemon chop
Mon - Doraemon chop
Tue - toys
Wed - Kitty slippers
Thu - Stricker book
Is she feel safe or want to share her belongings with other ? As she cannot take it to classroom, so I have to carry them to office and then to home everyday.
Is it strange for her behaviour ? Anybody have the same experience ? Let's share.