I'd really appreciate some help translating ENGLISH to LATIN

Hi all, sorry to just barge right in, but I'm trying to get something translated for a tattoo I'm going to have, as a sort of honour thing for my little sister, she has a complete bum of a boyfriend and I want to tell her I will be there for her, always.
Id like it in Latin so obviously it would be 'our' thing and leave her bum of a man completely in the dark.

Anyway what I'm after is something simple like "Brother, Sister, Forever"
Or "Sister I'll always be there", " Brother and sister for eternity" Something like that? I know you will be much better at language than me so if you have any ideas I would really appreciate it.

Thankyou

Paul