My younger sister got married before me. It was extremely hard. But it never occurred to me that she should ask permission or mechila. She had her own life to live. Who wants the pressure and the responsibility on their heads that their sibling can not go out?
If the 2 siblings are very similar and any name that comes up is equally nogea for both, then I agree the older one should go out with him. But otherwise it’s crazy to wait, especially these days when shidduchim is so hard for girls. There are older single girls who are probably still single because they didn’t date during their prime dating age because they were waiting for an older sister.