I am a fourteen year old boy and my sister is eleven. She needs help washing her hair so I wash it for her. Is this common?

It's not uncommon for older siblings to help younger ones with tasks like washing hair, especially if the younger sibling feels more comfortable with them or needs assistance. Many siblings help each other out with various things as part of their relationship. As long as both of you are comfortable with it and it's something she needs help with, it's perfectly fine! Just make sure that it's a positive experience for both of you.