SD9 is a bully...
A bit of background: SD9 lately has been exhibiting some interesting behaviour. Over the summer, SKs went to spend CO visitation time with BM and BM did one heck of a job manipulating SD9 to the point that she is a whole different child. No longer happy and bubbly. SD7 has told us that while there she was treated like garbage and SD9 was treated like a queen. SD9 was allowed to do whatever she wanted and would not get in trouble. SD7 on the other hand would get in trouble for everything. Now she is sneaky, manipulative and, lately, a big bully to her sister (SD7). She will pick on her at school and gang up on her with her friends, leading to SD7 coming home crying a couple of times. She says shes afraid of losing her friends if she doesn't do what they say. These same friends at one point backstabbed her and who was there for her? SD7. DH has been consequencing her by grounding her and taking away privileges, however she doesn't seem to get the point and continues to be mean to her sister.
Today SD9 let SD7 know that she will no longer be partnering up with her on twin day (which she had promised she would), and instead will be going with her friends. Essentially, she ditched her sister again. Now SD7 has no one to partner up with.
I feel that this is SD9 reflecting BMs behaviour along with not happy with being here loving with us, as she does not get the freedoms that her mother gives her. BM treats SD9 like a best friend rather than a daughter. On top of that, we recently found out from SD7 that BM has told SD9 to listen in and report to her. DH has told her that she is no longer in the circle of trust of our family since she has overstepped boundaries. SD7 stated that she refused to let BM know about what was going on which makes sense as to why BM treats her badly. SD7 brought up the bullying to BM and she basically just shrugged it off by "talking" to SD9.
Is this sibling bullying normal for this age? My sister and I never bullied eachother however we were 5 years apart.. How can this be stopped? I can see that DH at this moment feels frustrated and lost. Any advice?