Sparkly, this is probably the most important question you have asked on the CR, and it is really hard to answer it without knowing you, your parents, your older sibling, the reasons why your parents want you to wait, etc etc. I know people in this situation who consulted with their Rav, and then decided on a deadline- wait until x-(an age, time of the year, etc) to see if the older sibling will get engaged. If not, then the younger can start. This way the younger sibling does not wait indefinitely, but it is still respecting the older one.
while most non-chasidish girls will start dating after their year in seminary, at around 19, or 20, 21 is not that much more. You had a head start since you started college younger than most, you have been feeling like an adult for longer. I know that 21 can feel very old- I remember feeling very old when I was 21 and not yet married- but objectively, it is not. Starting to date at 21/22 is really not that old, only slightly behind everyone else, and you would have the advantage of having finished the first stage of your education, and bringing a more mature form of yourself to the next important stage of life.
It sounds from what you are saying that your parents are not expecting you to wait for your sibling no matter how long, but just to wait another year. They know you better than we do, so maybe you should trust them on this. And if you feel that it is very unreasonable, then turn to another adult who knows you and your family well (not just a rav giving a shiur, but someone who you can really talk to) to give you an objective opinion and if needed, intercede on your behalf with your parents.
And by the way, while you are waiting to start shadchan-style dating, you don’t need to hang out with guys.