How do I avoid feeling rejected in this situation?

Recently, my boyfriend's sister moved into his apartment. He comes from an extremely traditional family, as do I. So, when she began to take care of him as she did when they were growing up (cooking for him, doing his laundry, etc) I couldn't help but feel rejected and as if my job had been usurped. I tried to explain to him that women have the same hierarchy men do in family structures and that i feel like I am being replaced. He umed that I was attacking his sister and instantly became defensive and angry. I can understand his reaction, and maybe I shouldn't be so paranoid about losing my position in the household. However, she cooks the food that he likes because she grew up with him (they are from mexico), and knows how to react to his ire, etc. These are still things I am trying to learn. How can I avoid feeling this way?