Literally Bratishka means "little brother" in Russian (Brat = brother) .
Bratish' (brah-tEEsh) is a short form of "bratishka".
You can refer to your younger bros as bratishka(singular) or bratishki(plural). The term is mostly applied to those younger than you; sometimes it can be applied to bros of your age, and it's rare to heard it said to anyone older.
The word is widely used in post-Soviet countries among Russian and non-Russian speakers.
Bramism is a group (self-proclaimed religion) made by a few, in adoration for Bram Stoker from the famed manga and anime (soon) known as Bungou Stray Dogs.
In short (and what we now know), Bram Stoker is a Dracula with half a body. He's Irish and he LOVES music. He got high on it after Aya, a young girl, put earbuds in his ears so she could make a deal with him.
1: "Do you know of Bram Stoker?"
2: "The author?"
1: "No, the Dracula."
2: "What?.. Bram Stoker *wrote* Dracula."
1: "No! No, you've got it all wrong!! I meant Bram Stoker from BSD."
2: "OHH! The Dracula who got high on music! I do know him! Some people made a sort of self-proclaimed religion named Bramism."
1: "Yeah, I want to join them but rumor has it they're a bit... introverted."