Oi Lilly, Oi Diego
(May I call you by your first name?)
You have set me a real poser here!! I had to go and look in my English dictionary (which was of no help at all).
What I am going to explain here is not a grammatical rule or anything, so it is open to debate and maybe another English expert (or native speaker) could give you an "official" answer. Phew!!
For me the plural of "Sis" (pet abbreviation for "Sister"
could not be "Sises" (it would be pronounced the same as "sizes" and would create confusion). If the the final "s" is doubled in order to render the word plural - "sisses", this would be pronounced in the same manner as "cissies" (effeminate persons) and would also create confusion.
How would I say it? That is the question!! I have 6 sisters, so I must often refer to them as a group. Erm, when I refer to them individually I say "My big Sis", or "My little Sis", but as a group I almost always revert to "sisters".
I'm sorry, I'm not being of much help!!
Maybe we could ask kafetzou? Seeing as you have added the note under the translation, I think we can validate this one.
Thanks everyone
Bises
Tantine