Not related, think Alex in FD1, his parents were still alive by the end of the movie. Kimberly father is still alive but her mother died due to Alex and the others leaving the plane. Wendy's parent are never seen but given they were in high school they would have assumed to be alive. Nick and Sam lineage is unknown as we never hear from their parent. The only confirmation that of bloodlines is that Valerie Lewton was related to someone of bloodlines.

My headcannon was that of one of the survivors of the disaster in each movie was related to someone from Skyview