"You should help your grandmother."
Translation:Powinnaś pomagać babci.
In Polish Powinieneś pomagać babci would be virtually always understood as ‘you should help your (and not anybody’s else) grandmother’. English needs possesive pronoun here, where Polish doesn’t. On the other hand, if the talk was about some another, specified earlier, old lady (and every old lady actually might be called babcia), then the meaning would be different, and perhaps the most generic translation would be: You should help the grandmother (the old lady).
You mean "powinnaś".
"powinieneś" is what you will say to a man
"powinnaś" is what you will say to a woman.
This is a defective verb, it doesn't have some forms (like the infinitive, for example), it's hard to even say what tense it is: for example English wiktionary says it's past, but I wouldn't be so sure. But on the other hand, verbs in past tense have in Polish a difference between genders, so that is some explanation... Anyway, see here.
Because of the typo, which made the word "babci" (Acc./Gen. of "grandma") into "braci" (Acc./Gen. of "brothers") ;)
Otherwise, it's correct. You are talking to one guy, who has at least one sibling (or a cousin with whom he 'shares' grandma), therefore you may use singular "powinieneś" because you're talking to one person, and grandma can be "wasza" because she 'belongs' to more than one grandchild. That's not a usual usage, even then one would be more likely to say "swojej babci" or "twojej babci", but that's surely possible to say.