Hi Jenna!
I'm Dana - 32 yr old - single, no kids. No biological nieces and nephews. I have one sibling and she has said most of her life that she didn't want kids. So I get it - I have struggled with the same thing. I got tired of waiting and waiting for my own love/families, so I declared my spinsterhood and call myself an official third wheel. It's easier to accept this as my life (and find some humor in it) and then will be pleasantly surprised if I do fall in love or have my own kids.
I'll be honest - it doesn't necessarily get better when they are 8. There is less talk of breastfeeding, but they are busier with family activities.
I was very open with my nearest and dearest friends about my desire to be an aunt and my sadness over not having my own family. 2 of my friends have started to have their kids call me Auntie, and I am now included in birthday parties, family activities, cheering them on at sporting events, etc. I also take each child on a special date once every few months.
My sister and her husband have now decided to to adopt out of foster care. I am so very excited to love these unknown children, but I still feel the sadness of not having a genetic relationship to these kids. They also live across the country from me.
The best way I have found to ease that sadness is to choose to love and spoil other children in my life. I started watching a little girl every other Saturday (her mom is single, no dad in her life, and her extended family doens't help much). The relationship with that girl is a stronger bond than I would have with any biological nieces/nephews living across the country (if my sister were to have her own kids).
You are not alone in this! :) Talk to your friends. Let them know how you're feeling and, if you're not already, ask how you can be a part of their families lives - babysitting, attending sporting events, take their kids on special dates, etc. It sounds like you have a lot of love to give!