JMO, for what it’s worth – younger siblings should not have to lose out on their happiness just because their older sib was not as fortunate to find his/her bashert yet. My younger sister got married five years before I did. My youngest daughter got married before my older daughters who are not yet married. Had she not done so four years ago, I would have been deprived of the pleasure of my two year old granddaughter’s company every day.
That being said, I am also against allowing a younger sibling to date at a very early age (i.e. 18), when there are older FEMALE sibs in the parsha who are not married. It won’t kill the younger sib to wait another couple of years to give the older one a chance to meet someone first. I know from experience that as happy as you are for that younger sib, it is painful to deal with all the thoughtless commentary by people who don’t know what “tact” means.