I was born with hallux valgus (bunions) on both of my feet. My identical twin sister does not have this deformity. How can this happen?

Strange. Yes, bunions can have a genetic component, especially juvenile bunions; however there are many outside factors that can cause bunions. Does your sister wear a different style shoe, or have a different gait pattern? If you were truly born with them and had bunion deformity as a small child, i agree it is strange that your sister did not as well. Maybe one gene mutated? Genetics is not my field.
Genetic trait, but. Not all the children will inherit the trait. I must admit that if you are identical twins this is rather unusual.