I think you are correct in feeling angry.

Your SIL knew how you both felt and this is shown in how it was given to your son. They waited until they were on the way home. There was plenty of time during the weekend for it to be given to him.

She sounds like the SIL who will not respect your view even when you confront her.
In future if anything else is offered let her know it must be given to either yourself or your DH or it will be given back. That way you have control of the item/s and can give them back without any effects on your children.
In future, if things are given directly to your children tell them you will have to give it back as her children must have accidentally told them they could keep the item. This will also stop her off loading stuff she wants to get rid of

As for now, limit the time they are allowed to use the DS.
Don't stew over what is done as it is only affecting you not her.