You have SO many great suggestions. Just came to add one I haven’t seen.
Bear Alexander and Brock Sebastian would make cool brothers, ( Or any 3/4 syllable middle.)
Brock’s meaning is badger. Another possibility is Phelan meaning young wolf.
For a girl, the first thing that comes to mind is Bronwyn. Bear Alexander and Bronwyn Elizabeth
Bear and Wynnie. Oops, I forgot you want to use Margaret. Bronwyn Margaret flows beautifully.