Eh, could be worse. If she likes MLP you can subtly redpill her by drawing corollaries between MLP and the real world; it is a very conservative cartoon after all.
As for the OC, the only thing that can help is bleach, poured either on the eyes or on the alicorn. As a little girl she thinks that the more colors and features there are, the better. Cut down on stars but make them a symbolically important design feature, get your sister to decide among the three ponies (alicorn is way too Mary Sue-ish), cut down the colors to have them be aesthetically pleasing, show the cutie mark, and FOR CELESTIA'S SAKE GET RID OF THE ZEBRA STRIPES.
Well, Monet, it's all up to you. Fixing the OC is the easy part; the hard part is not making your sister hate you for "ruining" her pony.
How old is your sister? Because if she's older than 12 you should probably do what >>5393
said and save her the trauma of finding out the hard way that her OC sucks.
If she's 12 or younger tell her it's a very pretty pony and she did a good job.
Can you draw?
also is her cutie mark "KYS"?
Rainbow Dash is an example of it done right. She's sky-blue with a rainbow tail and mane, it's symbolic of a clear blue sky with a rainbow in it. Rainbows go from point A to point B, like a fast flier.
Also that glitter-coated batpony looks like ass. It's nice to see one that isn't purple with yellow eyes, but it still looks like ass. Where's the symbolism in its character design?
I already redpilled her. My sister won't be taken from me by the jews, not on my watch.
is your sister autistic or is she just young?
It's supposed to be a bunch of blue snowflakes.