Up until the astronaut part, I was fully convinced that this is a law school theoretical question for an inheritance class, because that's exactly where the vagaries of "is she my sister?" would also arise.

Then again, if we include time dilation due to near-lightspeed travel, we then have to deal with oddball inheritance cases like if your sister dies mid-travel but then you also die. The Uniform Simultaneous Death Act adopted by several US States would only apply if the difference in time-of-death is within 120 hours, but the Act is silent as to which reference plane will be used, especially if your sister is considered to be traveling "internationally" due to being in space, thus not being in the same US state or time zone as you might be in.

So maybe the entire question is a valid inheritance case study after all.