Yes, but you see it is like a title. Hi, sis. Well done sis. Sis is a teacher. Sis went to London. It is informal but common. . It would seem a lot more informal, & probably less common, to say "My sis went to London." --> "My sister went to London". "My sister is a teacher". , As with many informal things, I suspect you will find it more common among young people.

Hi Olga. Yes, "sis" is actually the short form of "sister" in this sentence.

Yes, just like Bro...