no: it is not the usual way to express it. but I have heard some natives express it like that
the sister who is 21 is the younger sister to you, and the older sister to the 18 year old sister.
the sister who is 18 is the younger sister to you and too the one who is 21.
you are the older sister to both of your sister.
the sisters who are 21 and 18 are the sisters "after you" but they are your younger sisters, not your elder sisters.
Therefore I believe the other people have misread your question. You can say that both sisters are sisters after you.
"they were born after you" but it is not a very common way of saying it.
older sister =you
middle sister = 21 year old
younger/youngest sister = 18 year old
or you can say smaller sister than me or
bigger sister