According to deer biologist Valerius Geist, mule deer/whitetail hybrids are not biologically sterile but they have behavioral issues that make them vulnerable to predation. They neither stot like a mule deer or escape by running with the head low like a whitetail. Thus, they don't have an effective predator escape behavior. As a a result they are more likely to be killed by predators and are less likely to survive over time.
They also have other behavioral differences that make them less likely to breed successfully with either whitetails or mule deer.
I've never seen mule deer and whitetails in the same group. It could be possible though I suppose.
