How to phrase a sentence with reference to "Least Superlative"?

For instance if I have two younger sisters, named Alice, and Becky, for example. How do I describe the sister who is not youngest, but the other younger sister?

Becky is my youngest sister. Alice is my next youngest sister? Least Young? Sister of penultimate youngness?

It gets worse if I have a third sister (Carly), whose age is between mine and Alice's. Now I need to refer to not only the sister who is older than Becky and Alice, but younger than I am.

Or is there no succinct way to express this? I tried to explain this concept to a French teacher once, but struggled to express it in English, and it's bothered me ever since.

Edit, to clear things up. I'm male. There's 3 sisters, and one brother.

Alice is my next youngest sister works if you have already said Becky is my youngest sister, while Alice is my second youngest sister works on its own.

Note that youngest does not imply younger than me. My youngest sister is older than me but younger than our eldest sister.

Of course penultimate means last-but-one, that is, "coming next-to-last in a sequence", so one could say that Alice is the penultimate youngest, and Carly the penultimate oldest sister, but both of those are clumsier than "second youngest", "next younger", and so forth.