I didn't become an aunt until my mid-20s. For years, I watched as my friends and cousins welcomed nieces and nephews in the world. I envied them when they spent their Friday evenings babysitting and when they attended family birthday parties and hosted sleepovers.
A few years ago, I finally joined the my-nephews-are-cuter-than-yours club (no nieces, yet.) While it has been everything I hoped it would be and more, there are a few things I wish I would have known before adding the honorary aunt title before my name.
1. Don't compete for the favorite aunt/uncle position
There is no predicting which aunts and uncles your nieces and nephews will like the most. Sometimes the one that loves kids wins. Other times, it's the curmudgeonly one who would rather stay away from kids. All jockeying for the Most Loved Aunt/Uncle position does is create discord among family members and in-laws. It may even annoy the ones you are trying to impress most.
2. Don't be offended if they spend more time with the "other" aunts and uncles
It is so easy to forget that your nieces and nephews have another set of aunts, uncles, cousins, and grandparents. This can be difficult to get used to, especially if you are unmarried and haven't had to deal with your own in-laws, yet. Cherish the time you have with your nieces and nephews, but recognize that others love spending time with them just as much as you do.
3. Respect the parents
You will love those little darlings to pieces, and they will likely adore you in return. It is oh so tempting to "corrupt" those little ones; whether by feeding them chocolate right before bedtime or teaching them to play baseball when you know full well their parents want them to play soccer. It's important to respect the parents' wishes. You may not agree with their parenting tactics. But unless the safety or well-being of the child is at risk you really have no say in the way their kids are brought up. Your job is to support the parents and love the kids.
4. Families with small children don't have flexible schedules
Your childless siblings might be able to go to a movie with you at the drop of the hat, but once they have kids that will change. They will most certainly want to get out of the house from time to time, but be mindful that even a simple outing to the movies requires careful planning.
5. Be around, and not just for the important events
The only way to establish a relationship with your nieces and nephews is to spend time with them, even if you have to go out of your way to do so. It is much easier to become the "stranger" aunt or uncle than the one the kids love spending weekends with, or talking to about things they are uncomfortable bringing up with their parents. Aunts and uncles have a special role to play in these kids' lives, and it's worth the sacrifices to build up those relationships.