I would recommend regularly holding her on your lap and letting him bounce around on the floor, reinforcing that she is the higher ranking family member. If she resents him they may not become friends. They will have to work out the pecking order for themselves but it sounds like she just needs to know she's not going to be second place. If she is confident in her standing she will be more likely to accept the new boy. It will take a little time for them to adjust to one another. Best of luck.
