Okej.

Imam dve sestre (we use "sestara" starting with number five, jedna sestra, dve sestre... pet sestara, šest sestara, deset sestara...) i da, uvak sam se svađala sa starijom. (There is no need to repeat the word "sestra", since we know you're referring to one. By saying "sa starijom", you're telling the reader which sister you're talking about. Also, we use "svoja" for possession. "Sa svojom sestrom", not "sa mojom sestrom".