Why do I feel this way?

I am in a relationship with a woman and I am convinced that she is the one for me. She is everything to me. There is one problem. She has a god sister, now the god sister who was in her care has two kids. The god sister disappeared without taking her kids. These two kids treat me like an outcast no matter my best efforts to try to bond with them. I am out of tries and I don't have the energy to try to make them feel as if they are my own anymore. Am I wrong for feeling like this?