We got married on my brother's AND my very best life long friend's birthday. We told them we picked that date in their honor.
Both came to the wedding and neither seemed to feel that they were being disrespected in any way. But I realize every situation is different. Just because my
brother and my
friend didn't care doesn't mean that it might not be extremely important to other people.
Having said that, I doubt if the OP's sister chose that date specifically because she knew it would interfere with her sister's plans for her very special 50th birthday celebration. Or maybe she did?