My Nieces

I have 6 nieces. Only one is related biologically, one is my husband's brother's kid, and four belong to my best friend, who's like a sister to me. The two I'm technically related to live near me, and my BFF's kids live with her in DC. I would kill and die for all of them, genetics be damned.
I see my youngest niece the most often, simply because I know there will come a day when I can't see her any time I want, and sometimes I get into moods where I miss my other nieces or I mourn not being able to have children of my own. She and I have a great time together. Her face lights up when the door opens, and she's just a hoot. I get my kid fix with her and go home with my heart bandaged for another day.
Maya is my husband's niece. She'll be 10 this year and is a very interesting child. I try to see her as often as I can, but until I quit my old job, it was more difficult because her family's schedule and mine didn't match on any level. Now that I do get to spend some time with her, I'm conflicted. She's a good, smart kid, but she's also a brat.
The other week, she came over and I made dresses for her for school. I took two days out of my crazy schedule, opened them up and made whatever dresses she wanted out of any of the 6 tubs of material I had. When I did this for her a few years ago, it was fun. She giggled, dancing around in each new dress and gushing how nice it was for me to do this for her. This time, I had to yell at her to come try things on, hold still so I could see what adjustments I needed to make, and I contemplated throwing her cell phone in the toilet.
I remember wondering what happened to my sweet, giggly, respectful niece?
Is this just something that happens when kids get older? Is this a pre-teen thing? Do all kids go through this? How are parent's not murderers, alcoholics, or addicted to anti-psychotics? How do they cope when their children become these things?
I LOVE my nieces, ALL of them, but lately I've began to wonder, am I the lucky one for not having children?