I hope you know how big of deal it will be when my niece and nephew decide to call you Uncle. It means they look up to you, it means they love you, it means they depend on you to be there for them when they need you most. Being an Aunt is one of the greatest feelings in the world and I hope you feel the same about being an Uncle.
I'll never forget the first time my niece called my last boyfriend "Uncle". It was the sweetest thing in the world and made me tear up. I loved how much she looked up to him and I loved the relationship that the two formed. However, when we chose to split he dropped her like she was nothing. He made promises to continue to see her, to take her fun places, and most importantly to still play an uncle role even if he was no longer with me. But it was always just excuse after excuse. I watched this little 6 year old girl at the time, cry wondering why her uncle was no longer around. Try coming up with the words to say to that... how do you explain to a young child why someone up and walked out of their life? I watched this tiny human ask to call him day after day just to say Hi Uncle ____, but time after time get no answer. Or for him to answer a make yet another promise that wouldn't be kept like bringing her the frozen bike we had bought her. No worries, the rest of the family stepped up and bought her a new bike but still you could see the disappointment in her eyes when he never showed. I remember when we first split how selfish I was, how I didn't want my niece or nephew to have a relationship with him. Luckily my nephew was young enough where he'll never remember him but my niece was plenty old enough to remember everything. It took some time, but I learned it was not fair to take my niece away from her uncle, but little did I know he would just leave her as he did me. It's sad that my niece's first heartbreak had to come from a person that should have defined unconditional love to her. I get life changes, but somethings should always remain true.
So, I hope you do not take the responsibility lightly. If this isn't something you're up for then please do us all the favor and don't come around them. Being an uncle can be one of the greatest joys in life. You get to take the kids to do fun things, get them all sugared up on candy, and send them home. The joy you'll get by them lighting up when you walk into the room is one that will stay with you, always. They'll want to be just like you, they'll want you to come to all of their sports games, school events, and family things. In fact, they'll probably ask where you are when you're not able to come. Being an uncle is not something you can just walk out on, well... At least it's not something you should walk out on. In the end, it's the children's heart you'll break, not mine. And if and when I chose to start bringing you around them, please no how big of a deal it is. They are my world and I would never want to put them in a situation that could hurt them in the end.
Lastly, I hope you realize it's not them that's lucky to have you, it's you that's lucky to have them, because they are 2 pretty amazing kids.