I felt the need to reach out when I read your post. You aren't being insensitive, you are being honest. Don't feel bad about that, you have outlined how you feel including all the thoughts you have about feeling like you are cruel. Why people are taking this as an attack, I don't know. You are just being honest!
Anyway, I feel the same as you. I have a 6 year old son. He has 2 cousins nearby and gets upset that they have each other and he is alone. We moved and part of the reason was to be closer to his cousins. We got a puppy to add a bit more chaos to the house as well. A much wanted addition I may add!
We tried for a year to have another child from when our son was between 1 and 2 years old. It never happened. We decided that we can go get tests done or we can be grateful for our one child. We decided on the latter, and for a while it was fine. Now I feel it was a mistake to stop trying. Like I'm a failure and selfish. These are MY feelings, not how I view other one child families.
I'm guessing you suffer from low self esteem and need opinions from other people (usually your husband?) that you've made correct decisions. I know I do. I don't trust my own mind. Are you idealistic? Do you have trouble accepting your life as it is, rather than the perfect picture you invisioned? That's me, but you may or may not be the same.
As parents, we want the best for our kids, to feel we have provided everything they need. I think that is natural. With not providing a sibling, we feel we have robbed them of a life experience. The guilt seeps in. And it can't go away because the age gap is too big now to provide the 'best friend' experience. So we torture ourselves and worry our kids will hold it against us in the future, that we didn't provide for them.
It's ok, I HEAR YOU. Maybe my account of this isn't quite the same but I just want you to know I understand and I'm here, listening.
Our kids will still have a great childhood. The same as so many only children. She has a best friend and that is wonderful that you are fostering a great friendship between them. Keep doing this.