Wow - blended families are a challenge! I have several friends in the same situation, except that every other year, they always have the kids the whole day - Christmas Eve to Christmas night - the dad has them one year, the mom gets them the next and they work out the rest of the kids' Christmas break however fits their schedules... sounds like your hubby may have an unfair custody situation going on...
Anyway, I agree with what most people have said - open the "family gifts" when the whole family is there - especially gifts from her to him and vice versa. Santa can be done whenever - but I would never expect a child to WAIT until 3 pm Christmas day for all her Santa presents. Maybe you could stick a few aside that she doesn't see at all until he is there, and then tell her there are a few more surprises Santa left to open with her brother. This will be especially important as they might start sharing things when she is older. For example, my neighbors have a 2 year old son together, 2 kids that are hers, and the oldest that is his. They got his daughter (lives with them every other week) AND her two kids (with THEIR dad every other week too) an Xbox last year that was for all of them to use, so they all had to be together to open it, which didn't happen until noon. So the two younger ones were at home that morning, but couldn't open any of the games that went with the system, so they got all their other Santa stuff first thing, then knew they had a HUGE surprise for when big sis came over - it was covered in a huge Christmas blanket - as were all the games - individually labeled unless they were "shared" games. It seemed to work well and the kids got just more and more excited b/c they all 3 knew there was something HUGE and it really helped them to bond, since the "other" houses didn't have a game system at the time. Now of course, the older girl has a Wii at her mom's that is ONLY hers, and the other two have a PS3 at their dad's, so I guess they get the best of all worlds...
Most importantly, just remember to teach the REAL reasons we have Christmas and not the superficial, material ones. Kids will remember fun and important TIME more than they will remember certain toys/gifts when they are grown...
God Bless you and your families! :)