There are some issues with punctuation.
So there I was, sitting on Santa Claus lap right next to Wendy's' hospital crib, whispering quietly into his ear, "I don't want any toys this year, I just want to go home with her."
The bolded part is makes the sentence sound a little awkward. I would reword it to something like.. "When the hospital Santa Claus asked what I wanted for Christmas, I whispered quietly into his ear "I don't want any toys this year I just want to go home with her". The period should be after the quotation marks, I believe.
Because my sister isn't like everyone else I have stressed the importance of acceptance towards others and value respect as an important characteristic in a person.
I would reword "because my sister isn't like everyone else". Maybe change to "due to my sister's condition"
I would try to tie in the first paragraph with the second. There is too big of a gap from your sister and how she has changed you directly into you in high school. I would save this part...
This helped me realize how much I've grown and changed mentally as a person. After a successful battle with my sisters Pneumonia, I'm thankful my family and I were given the opportunity to raise a child with an intellectual disability. Wendy's happy and cheerful character is always there to make people smile and has made me a better person. Because my sister isn't like everyone else I have stressed the importance of acceptance towards others and value respect as an important characteristic in a person. I have accepted my responsibilities as an older sister and know that the focuses in my life won't always be about myself.
until the end of the essay.