For sure they are old enough to be sat down and given the house rules. Remind them that no matter what the rules at their house or grandma's house or their friends' homes you have rules at your house they must follow.

At the same time I do tend to be a little bit more lenient with siblings when I can in the sense that they are playing as they have learned to play and there isn't another parent that is going to complain about the agression towards their child. But that doesn't mean shoving, etc. is acceptable period.

Talk with them about what the consequences should be for forgetting the rules at your house and then make them stick to them - they will delight in reminding the other one about needing time out or whatever method you use. They will learn to police themselves with time and all you will need to do is give a few verbal warnings/reminders.