god first, family second

It's hard being the only Christian in my family. My sister may say she is, but unfortunatly, she doesn't really get it. I don't really get it, but I'm actually active in trying to understand and relate with God. It's especially hard because Em and my brother in law are flying back east for christmas, cashing in most of their vacation days from work, and it's a christmas present from my folks to them. Herb wants to see his family for Christmas, and then they're driving to roanoke for the days between christmas and new years (well, more like they're getting a ride half-way and will need to be picked up somewhere between northern VA and roanoke). I didn't tell them until I knew their flight dates that I was going to be at Urbana (big, once-every-three-years missions conference with IV) for the second half of their trip. I also couldn't really delay in signing up for Urbana because of the $75 jump in price and the fact that even if I had waited, it wouldn't have changed the fact that I feel God's calling me to go.
My sister did not take well to the news that I was going to a conference while they were back east. I could pretty much hear how angry she was through the email and she used some strong language to convey her feelings. I tried to explain to her some of the more spiritual reasons I have for attending Urbana (God's calling, being equipped to be an effective witness in my life ahead, etc.), because apparently the simple logic (it's only once every 3 years, I don't know where i'll be in three years so this may be my only chance to attend this conference) made her feel like i was choosing to spend time with my friends over time with my family. She glossed over my suggestions to get around the issue of my being away when they're in Roanoke, so I retiterated that I was willing to drive to Northern VA to see them, even plan to shorten my time in Roanoke before Christmas so that I would be in northern VA when they arrived. I can only hope that that's enough to help Emily see that I do care about her and my brother in law and I do want to see them, it's just unfortunate that I'm not going to be available for half their visit.
Now, part of me, in the back of my mind is going "why can't they come to Roanoke for Christmas? my parents are paying for the tickets, not Herb's parents, why can't we get them for Christmas and they can spend the other half in northern VA. Why can't they be flexible in their plans? Am I not worth enough for them to simply swap the order in which their vacation is planned?" But I know that's not really good thinking. It's not the way Em and Herb work, Herb's made up his mind, so that's what they're doing. It's rough though, because I just don't know if Emily will ever understand my "God First, family second" mindset/worldview. I don't know if she'll understand the idea that God could actually be calling me to Urbana and that being able to live my life for Jesus while still having a 'career' of sorts is important to me.
i love my family, but it is so frustrating sometimes.