Ellie, I'm so sorry your family treats you that way. It's so disrespectful and degredating. I don't even believe that (actual!) children should be talked down to.
YES! There is something to this!
For me, I feel like it's both about being treated as if I was younger AND about having my development stopped so that I COULDN'T grow up. What you said about your nieces really resonated with me. It sounds like they have had their emotional growth stunted so badly. I'm sorry that they act so horribly, but I can't help feeling sorry for them.
I'm in my mid 20s but I feel like I'm about 14. In terms of life experience and personal development, I may as well be 14. My mother halted my growth starting when I was around 8. As soon I started to have interests that differed from hers - BAM - she put her foot down hard. She punished me and treated me like a horrible child for wanting to do the normal things that kids do to develop their own identities. Clothes, music, make-up, boys, tv, movies, friends... all bad. She squashed me. It led to me having an emotional break down at the age of 10. I never really recovered from it. Afterwards, I just gave up on everything. I didn't even allow myself to want anything anymore, because I wouldn't have gotten it anyway, and she probably would have made me feel badly about myself for it.
My therapist says that my mother was always competing with me. She felt too threatened by me to allow me to be a woman. So she kept me as a child. That way, she always won.
For an example, when I was about 16 my mom and I were shopping for a formal dress for me. There was a strapless dress, but I said that I would never wear something like that (after years of my mother telling me that I was fat -- I thought I was way too fat for that dress and would make a fool or myself for wearing it -- not to mention that she'd always freaked out at me if I showed an interest in anything sexy or grown up). She replied by saying "well, I would wear it, because I'm more daring than you!" I just started crying. I was so confused. She always told me how shy and reserved I was and I hated it -- because she'd forced me to be that way! It hadn't even been about being daring or not. It had been because I thought I'd look ridiculous in that dress because I was too fat. And I knew that what she said wasn't even true - she wouldn't have worn it either. She wasn't the least bit daring. And she really was too fat for it too.
N just dread the thought of aging and mortality. If your mother and your sister thought of you as a 44 yr old woman, they would have to think of their age.
Oh yes, this is very true too.
I finally yelled to get her attention and said - but aren't you proud of your grandson? She mumbled something I couldn't understand and went on to the next subject she could use to belittle me with.
Good for you for saying something!

Sounds like classic N behavior. Of course, if you're bringing up any topic, it's only so that she can talk about herself! From listening to my mother, I think that's the only way they even know how to have a conversation. It sounds like she might be competing with you like my mother does with me.
Yuki