My 12 years old son hits, pushes, and teases his 7 years old sister.
When my daughter was born he would constantly disturb me while feeding her and helping her. I tried to involve him in taking care of her, but it did not help. I took him to my pediatrician and she spoke to him and he was fine for few days. As they grew older he started taking her toys and pulling away things while she was engaged in any activity. I tried to take away his privileges and he would show temporary improvement.
At school he behaves very well and his teachers appreciate him. His grades are consistently good and he works hard to excel in academics. My husband is only concerned about his studies and says that all siblings have this type of clashes. When my daughter complains about my son, my husband shouts and threatens him. This does not have any effect and my son says that she provokes him and she should be blamed for his behavior. I am confused and frustrated.
Looks like there is a lot of jealousy going on which has not been resolved for a long time.
I suggest reading Siblings Without Rivalry. It addresses different scenarios and gives suggestion on what to do in different situation, including the ones you described above.
Meanwhile I suggest:
Don't react to your daughter complains the way your husband does(and this is discussed in the book as well). She might have figured
out that it gets her brother in trouble and use it. If not now, then
in the future. If you did not see what happened you can't assign the
blame. If your daughter complains, tell kids to resolve the issue
themselves. If it does not get resolved both would face
Talk to you son and spend some time alone with him. Tell him you love him. A lot. Do the same with your daughter. But not at the same time.
Don't put kids in the roles. Older brother does not mean more responsible and protector. Younger sister does not mean weak and blameless. He does not have to love his sister (though this is something we want as parents), but he has to treat her with respect.
In the spirit of SO, I'd like to ask what have you tried ? Anyway, I am assuming that you tried to explain gently and politely and that did not work. However, if I were you, I'd try to do it like this:
No one like to be mistreated. Would you like it if someone bigger and stronger did that to you for no reason at all ? Go hug your sister and say sorry to her. Tell your daughter to be prepared for this and accept his apology and love. If this also does not work, remind him again and gently push him a bit 3-4 times to show how it feels.
Let us know how this goes.
She should certainly not be blamed for his behavior, but (speaking from personal experience) it is very possible that she does provoke him. They will both grow out of it eventually, but in the meantime, if your daughter indeed does or says things to your son that are not nice, put some blame on her as well (although obviously less blame than for hitting). I don't think you can expect a 12-year-old to keep his cool under constant teasing the same way an adult does.