I have no brothers or sisters, but I have many dolls.
I'm always watching my mother, and my cousins all think I'm fat.
I have no brothers or sisters, but I have many dolls.
I'm always watching my mother, and my cousins all think I'm fat.
I think you are
The moon
I have no brothers or sisters, but I have many dolls.
The moon is the only natural satellite of earth, but there are many man made satellites orbiting the Earth with it.
I'm always watching my mother
Tidal lock.
And my cousins think I'm fat.
Maybe the stars? Cousins in the fact that they are visible in the night sky, but not made of the same stuff. The moon is fat because (even though the stars are physically larger) it appears far larger than them to someone standing on earth. Or maybe because it is made of cheese.
You are a
Babushka doll
I have no brothers or sisters, but I have many dolls.
The Babushka doll is composed of many layered dolls
I'm always watching my mother...
Every doll is facing the doll that encloses them
...and my cousins all think I'm fat.
Every doll gets bigger as you go out
Are you..
Ganymede ?
I have no brothers or sisters,
Jupiter's moons, including Ganymede, were all most likely captured from solar orbits
But I have many dolls.
Several Jupiter-bound spacecraft have been able to observe Ganymede
I'm always watching my mother,
Ganymede is in a synchronous rotation, similar to the Moon
and my cousins all think I'm fat.
Ganymede is the largest moon in the solar system
Are you
Jupiter?
I have no brothers or sisters, but I have many dolls.
The 60+ moons of Jupiter
I'm always watching my mother...
The sun
...and my cousins all think I'm fat.
Jupiter is big! The cousins are the other planets.
lateral-thinking Are you
A creepy serial killer with a particular family history?
I have no brothers or sisters, but I have many dolls.
You are a single child who received inadequate affection from your single mother and turned to a doll collection as an alternative sense of warmth. It is a hollow love, though, given by porcelain souls. Many are broken in places but still stare out at you from the darkness within their skull, their one good eye unblinking in its judgement.
I'm always watching my mother...
Your mother was your first victim and her remains are propped up in a basement chair. She is surrounded by the dolls and, as their souls are really just pieces of your own, she is always watched by you through them.
...and my cousins all think I'm fat.
You actually are slightly overweight and the few cousins you knew have teased you about it your entire life, further isolating you from happiness. They are all part of your collection now, too. Many dolls have been patched up by replacing their broken pieces with chunks of your cousins. A patch of scalp here, a sliver of heart there.
Of course,
You are happier now. You have your whole family with you. All the time. Always watching. Always. Watching.
Probably not what you had in mind, but it can match pretty well.
Are you:
Microsoft Windows Operating System
I have no brothers or sisters,
Windows has no brothers or sisters (but children [XP, vista, 7 etc] and cousins [Unix/OSX])
but I have many dolls.
Windows can have many 'dolls' of itself (i.e premium, ultimate, home, workplace)
I'm always watching my mother,
Windows is always watching/incorporating an aspect of it's 'mother' MS-DOS/Command Prompt (however windows itself is often much different from its previous generations in many other aspects)
and my cousins all think I'm fat.
It's Unix cousins would think Windows OS is 'fat' because of how much more space it required for basic installation(30+ GB). (Where as Unix can be under 1GB)
Cinderella
I have no brothers or sisters, but I have many dolls.
She had no brothers or sisters , only stepsisters. Dolls- not sure.
I'm always watching my mother, and my cousins all think I'm fat.
Always watching her fairy godmother who helps her .Her stepsisters were jealous of her and might amount to thinking that she was fat.