Why should I express my emotions in a family relationship?

Why express feelings. Relationships are best when they include sharing and trust. It's important that people in relationships can share who they truly are, and not hide or try to pretend they are someone else in order to be loved. Sharing feelings increases our sense of security and social bonding and makes our relationships stronger. This is important for long term happiness and health.
Sooner is easier. A lot of people don't, because it's your family! however, the more and longer you repress your feelings, the more likely these emotions will eventually explode in a way that's even harder to manage or recover from (both for you and your family). Expressing your emotions can also help your family member(s) to understand or at least be aware of what are the "triggers" for you.