I’m an older sister in a similar situation (except she’s 2 years younger) and I firmly believe that each of us will find our shidduch in the right time, and there is no point in rushing it or holding back.
In your case, since the younger one went to college, they are probably already in different tracks in life and looking for different things. It’s good to nip in the bud the feeling of keeping up with the joneses, everyone gets what’s meant for them.
But of course, ask your Rabbi / Chacham.