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.