I've lived with my fiancé for over two years, looking at a home to buy together, when she was killed in a car accident. Do I have any rights to carry out her wishes to be buried near me and my family which she said was the only real family she has ever had?

Does she have family? Had she written down her wishes?

I'm so sorry for your loss. You'll be in my thoughts.

You would have no legal rights, but would her family be amenable to including you in the arrangements?

I am very sorry for your loss. I don't want to go into details, but that burial with you and your family (and maybe future family) just doesn't seem like a good idea to me.
Her family has to be going through some hard times and regardless of what she thought of her own family, that has to be a serious blow to them. And I don't know how it will affect your future family.
Maybe if I knew some details about why she felt that way about her family I would feel differently but unless it was extremely out of line- No. Where are her remains at present?

I am very sorry for the loss, but no you do not have any rights. If she had a will, that gave details, but not at this point. Her family will be the ones who decide.

If she was estranged from her family, and they feel the same, then they may be willing to let you do so because it means taking on the financial burden. So just ask politely and carefully, and hope for the best. I wouldn't mention her wishes or money. Wishes can be assumed to be lies, and they might even get suspicious about how she happened to even bring it up. Just ask.