Ah, what a lovely sentiment you have there. To reference your sister in the phrase 'a friend is another self,' you can say 'alter ipsa soror.' It's wonderful to see you finding ways to express your connection and love for your sister through beautiful Latin phrases.
Amicus.
Tuus amicus/tua amica.
Amicissimus for a male friend. Amicissima for a female friend. Alternatively: Amicus carissimus for a male friend. Amica carissima for a female friend. These mean dearest friend.
Amicus was created in 2001.
"Amicus meus" means "my friend."
Project Amicus was created on 2011-03-24.
It means, "You are a friend." It could also be said as "Amicus tu es" but "es" (the particular form of "to be" implies the pronoun "tu" (you).
An amicus is a person in law who is not party to a case but who submits a brief and/or presents an oral argument in that case.
Amicus.
A sure friend in an unsure situation is the English equivalent of 'amicus certus in re incerta'. In the word by word translation, the noun 'amicus' means 'friend'. The adjective 'certus' means 'certain, sure'. The preposition 'in' means 'in'. The noun 're' means 'situation'. The adjective 'incerta' means 'doubtful, unsure'.
Amicus brief on behalf of 16 detainees held for almost two years.
Amicus de caenator/coenator. Amicus convivia Amicus = friend Amicus = guest de = of Convivia = people/guests at party Caenator/coenator = dinner guest Convivium = banquet, feast Caena/coena = dinner, supper General word for guest: hospes