feif on – i don’t believe that is only reason guy isn’t going out – though he may use it as his excuse.
the whole issue should be handled in a mentchleichtic (spelling?) way. meaning: younger sibling (and/or) parents should get permission from older sibling before going out. (older sibling should grant permission unless younger really is so young and no necessary reason for sibling to go out then, but wouldn’t harm anyone if waited a year). then when getting close to getting engaged, younger sibling should again ask older sibling permission. (obviously, older sibling will say yes – it’s just to show that i’m giving you respect and is that all right with you). by doing these 2 things, will prevent much heartache between siblings. both know that acted with other one with respect, and whatever else happens is from Hashem. when not done properly, it breeds resentment and estrangement, and neither party is happy for the younger sibling’s simcha.