Around here, the every-other-weekend parent has to be the one there during the exchange, you can refuse to give the child over to a grandparent, etc.
I understand that may not be the best route, for me it would be important that my stepson have a good relationship with his siblings, too.
It stinks, and I would be really upset if my stepson's mom did that, but I think I'd just be supportive for my boy and ensure that we stay positive and not start the baby momma bashing (at least, not until I was sure he wad asleep. lol)
Emily, Lovin' Life with DF Ben, "Em" to super-silly DSS Gage (04/06), and new mommy to Kai (08/07)