GH
I’m sure you read my original post where I called the practice of enforcing a child to wait for an older sister to get married imprisonment and evil.
My second post was an attempt to take feelings out of the equation and quote sources.
Here are two more I found
Igros Moshe EH 2:1
case involves two brothers. Rav Moshe says brothers dont wait bc there is a mitzva of p’ru u’revu. Says nothing about a need to ask mechila and says the younger brother has nothing to feel guilty about and should pursue the shidduch
Mahrsham 3:136 (Rav Scwadron, grandfather of the magid)
Also states doesn’t apply to boys. Brings down that it applies to girls as a din in derech eretz (in other words no one can force her to do this). his specific case involves a 21 and 27 year old girl and says she should pursue the shidduch offered bc who says the older will get married and why should she turn down her chance. My interpretation is that He certainly implies that every girl has a right to use her own judgement.
As for the OP who called it a “minhag”
Based on my research . For boys certainly not true. For girls there is a din in derech eretz for waiting a respectful period of time bf actively putting herself out there (to be determined by the girl) but if something good comes up she should pursue it.