Mid (?) Life Crisis

I am getting old. It is vain and cliche to be in this frame of mind, but I am.

Soon, my fertility will start to decline, if some terrible calamity doesn't claim it first. In fact, my fertility is probably already declining.

I have no siblings anymore, I am 'the last of my line' so to speak. This makes me sad. I grew up in a family with lots of cousins, aunts, uncles. My children will have none. If I have children., how will they learn the lessons that I learned from those relations, from having those relations and from growing up among a group of people who loved me unconditionally, (and learning to accept that there would be those that did not)?

Then again, what constitutes 'family' in the sense that I understood it in my childhood? I have friends that are like my sisters, and having none of my own, would they not then stand in that place for my children? Not having a 'blood' support system, wouldn't I as a parent be more driven to create such bonds with non-relatives?

How could I ever even compare?