my [young / younger / youngest] sisters

If I were introducing my siblings, I would consider that simply appearing to follow age order and winding up with "and the youngest is..." would be enough to establish age order.

Let me introduce you to my younger sisters. A and B are twins, this is C, this is D, and my youngest sister is E.

If you're including ages, like you did in your question, nothing further needs to be explained about their age order.

Let me introduce you to my younger sisters. A and B are twins - they are 12, this is C - she's 10, D is 9, and E is 7.

Note that there are a couple additional ways that my examples differ from yours: first, it is usual to begin introductions after a preliminary from one side, not right after "Hi". So your companion might say "Can you introduce me?" or you might say "Let me introduce you to my sisters", before embarking on the introductions. Second, "she is" is replaced with "this is", since "this" is more personal; "she" sounds like you're referring to someone across the room or on a stage, etc.

Is this conversational?

... and these are my younger sisters: a, b, c, d, and e.

As to the grammar I couldn't say. Looks ok as I read it. But I'd say, "this is", not "she is". I think it would come out something like this in my home town.

Hi, these are my younger sisters. This is A and B, who are twins. And these are C, D and E. D is ##, C is 9, and my youngest sister E, 7 years old.

I think "respectively" sounds odd, but that probably is just a regional thing.

I would say that A and b both are my eldest sisters and They are twins and both are 12 years old. C is my middle sister and her age is 11. D is my next middle sister and her age is 9. E is my younger sister and her age is 7.