I can assure you that I am in no way playing the parents off against each other. That is a huge assumption to make!
Another reason to run this past your SIL is to get a heads up about any issues between your brother and her wrt potential gifts - as an example, a father who will see boys EOW wants to get phones for DCs, mother doesn't want all the issues surrounding phones, internet access, porn, violent videos, sexting.
Are you sure there wasn't already some argument between your brother and his stbxw over fitbits? All the trendy kids have them in schools now, and it's not inconceivable that you have inadvertently stepped smack into the middle of an ongoing fight between the parents over them.
It's not inconceivable that a father might try to be Cool Dad and say yes to all sorts of gadgets when divorcing, just to spoil the DCs a bit, whereas a mother might want to maintain the status quo if she is going to be the resident parent, especially where electronics and current fads are concerned.
Since your brother and SIL are divorcing you are going to have to be especially mindful of this aspect of gift giving.