Normally, you would use "tu hermano", to mean "your brother".
"tu" is the possessive (your) for the personal pronoun "tú" (you).
It means the singular informal you.
If you want to use the singular formal you, so, the more formal "you", you will use Usted.
Usted means a you, but is conjugated as a "he" (or she), and its possessive pronoun is also the same than for he/she, it's "su".
So, if you talk about a "he/she": Ella es guapo, y su hermano también (She is beautiful and her brother too).
If you talk about a "tú": Tú eres guapa, y tu hermano también.
Is you talk about a Usted (polite "tú"): Usted es guapa, y su hermano también.
There's also a plural Usted, it's Ustedes. Ustedes son... It also use "su". Ustedes son guapas, y su hermano también.
"Tú" with an acute accent on u is the subject pronoun "you" (informal singular).
"Tu" without any accent mark is the possessive pronoun for "tú"—it means "your".
"Ti" is what is called a tonic pronoun. It is used after most prepositions (para ti, por ti, a ti, de ti, etc.). "With" ("con"), however, does not take "ti" and instead combines to form "contigo". Additionally, there are six prepositions that are followed by subject pronouns instead of tonic pronouns:
entre — between
excepto — except
incluso — including
menos — except
según — according to
salvo — except