awww, Not a burden if you don't push her towards it, just a sweet little girl wanting to help out! She's going to be a great big sister to the baby too!
DSD also has two sisters on her mom's side. One is older than her (1 and the other is younger (12-ish maybe?). The truth is that they don't really know DSD's life expectancy. There aren't many people around with her condition. Ok, they exist but not many older ones who are diagnosed. The condition couldn't really be diagnosed prior to the advent of genetic testing.
This is what she has: What is Phelan-McDermid Syndrome?