Chrisks, my situation is fairly similar. I have a daughter who is my only child. I've brought her up exclusively by myself as a single parent since she was 10. Her Dad moved on and didn't have much contact, nor contributed to her upbringing financially. She is now 31, married two years ago and expecting a baby quite soon.
She is really close to her in-laws, and they love her a lot. They are a type of family that is very different to what she has been used to. Very traditional, lots of family get-together's and make time for family holidays / breaks where the (adult) children, spouses and grandchildren are included. Her family experience has been me, with a couple of cousins thrown in. The in-laws live quite close to them and I face the reality of them being more close to my grandchild than me (I live a day's travel away).
You know, I think what you may be feeling is a sense of loss, perhaps on different levels. You lost your husband and your daughter lost her Dad. You had to be everything to and for her. There are so many layers in these experiences. I gave my daughter away at her wedding, and it was a beautiful experience seeing her so gorgeous looking forward to a wonderful life. She is so loved and so cherished. I made a speech about love, and felt the pang of what I thought I had not been able to give her. But I know I gave her all the love in the world, and that will be the same for you and your daughter.
I think it is such an honourable thing that you felt able to share your feelings here. Feelings are often on a spectrum. It is ok to reflect on what her 'other' family mean to her, and feel pangs of goodness-knows-what. For myself, it's been a process, but mostly I feel incredible gratitude she is immersed in a traditional family who will provide fantastic structure and relationships (even the Grans in their nineties are still going strong, so the wee one will have great-grandparents. My parents died when I was in my 30s).
Put simply, it's a different relationship your daughter has, but not comparable. It brings more beauty to her life and happiness, but never at the expense of your relationship and the love between you x