Relativity

I just realized I might have messed up the physics of relativity in my sequel. A character in Atmosphere and another in Ocean are sisters (not much of a spoiler, really). I was running on the assumption that since their departure and destination points were the same, the age differential when they met would be as well. I'm no longer sure that is the case. Chara, the destination in both books, is 26 light years away. Assuming an accelleration at .5 G, that would be a trip of about 10.5 years. With no aging for the crew (the "magic" of stasis), they would arrive at the same age as they left, say 28 years old for the one sister. In the meantime, the sister who remained on Earth would be almost 30 years older, say 39. If she then heads to Chara more or less when her sister arrived there, in stasis, she will be 39 when she gets there, but the sister on Chara will have experienced 30 years and will be 58. So the age differential is the same, but the plot of my book is screwed. I had the second mission arriving ten years after the first. For that to be the case, they would have to have left 20 years before the first arrived, Earth time.

I have a couple of problems here. I am wedded to only ten years having passed since the end of the last book. Can't tell you why without spoilers. I also need the sister to be on this second mission. And the second mission has to be aware of the events of the first before they leave Earth. At this point the only solution I can think of is FTL travel.