@NanoRaptor
Happy birthday!

Also, no, you can't work it out.

What we do know:

Your younger sister's 21st birthday is in the past, otherwise your older sister's date of birth would be in the future (an impossibility).

Your older sister's date of birth must be long enough in the past that, at a point where your younger sister turned 21, she was half the age she is now while (probably) being 23 or older herself at the time (assuming there is at least a

year between the siblings, a fairly safe assumption unless you are half siblings or you are triplets).

You could be triplets (in which case you would all be turning 42 today).

Failing that, assuming your older sister was 23 on your younger sister's 21st, she would now be 46, you would be turning 45, and your younger sister would be 44.

If your older sister was 25, she would now be 50, your younger sister would now be turning 46, and you would be between 47 & 49.
@NanoRaptor

@NanoRaptor
Of course, there could be off-by-ones depending on the exact birthdays, but those are details.

Also, I believe you and your younger sister could be twins, but you and your older sister can't be without also being triplets.