H.W.
I'm not a mother of twins - but am a mother of siblings.
I would say "It's not ok for the little boy to take the toys from his sister - also not ok for her to be aggressive."
I would interfere everytime he takes a toy away from her, otherwise, it will just continue and you will be mitigating the problem your hole life.
When he takes a toy away:
1. Take the toy back from him and give it to his sister.
2. Remove him from the play area.
3. Tell him "We do not take things away from your sister"
4. Let him sit on your lap, with no toys, for a few minutes.
5. If he cries, wait for him to settle down.
6. Tell him, "I'm going to let you play - do not take toys away from _________"
Repeat this every time... it will probably take a few days, but he will stop. Good Luck !