I’m extremely close with my older sister. I did not want to go ahead of her, but was told that I should start dating. (Many of my friends were married with 2 kids already when I started.) Within the year, my sister B”H met her husband. Two years “into the Parshah,” my younger brother came home from E”Y and was chalishing to begin dating. As painful as it was, I kept saying that I have no right to hold him back from performing the mitzvah.
My advice to parents with multiple children dating at the same time is to consider the feelings of all those involved. It is very painful, when after accepting the fact that a younger sibling may get married first, and everything is a “big secret” and “hushed” so that the older sibling shouldn’t “feel hurt.” The biggest pain is often the pain of not being informed and finding out “the scoop” from outside sources.