I have a brother and sister duo,they are 26 mos. Boxers do seem to play with each other differently,at least from what I've seen and read. My two play rough with each other,boxing,growling-you'd think they were in a WWF match

But they aren't aggressive in any way,never have been.
I'm definitely not an expert but at 9 weeks they are babies and still learning what things are. Life is one glorious party

If you think it may be getting a little too rough,if you have separate crates,put them in for a few minutes,like a timeout. The best advice would be to check out the "rearing puppies"forum.There is a lot of great info there.
Good luck with the babies,I remember well the fun we had watching them play
Sue and the kids,
Pooh,Bownser,Dozer & Cissy