Well I've got the same experience. On that day my husband and I were line up one in 88F & another one in 88G. When 88F came I called my husband to jump-in a lady told him to line up then I got-in first and held a seat for him. Then this lady scold me to release the seat for her. BUT I won't because my husband was getting up. At that time there were still few seats available.
I think if only one person (I mean relative) jump-in is acceptable. But can't be too many. Are you agree ?