I know nothing about dandy-walker, but the pp seems to, and since it seems to affect the brain primarily, I just want you to know that brain injuries/damage/other afflictions are not a death sentence. And it doesn't mean the child will have a horrible life. Of course, I'm sure your sis is devastated now. I was too when I learned of my DD's HIE. But it DOES get better. My DD was diagnosed with moderate/severe brain damage. At birth she didn't do anything and didn't have a single reflex. She couldn't eat or breathe without help. She's 6 months old now and still very delayed, but she's definitely "there" know what I mean? She smiles and coos and interacts some. Not a lot, but more than the docs thought she would. All of her reflexes are back, though she still needs to improve some.
When DD was 9 days old, they gave us the option of discontinuing her care...in other words, letting her die. I look back and I'm just amazed that those doctors think she was in such bad shape that we should have considered letting her die. Even though she was diagnosed moderate/severe, I really feel she's going to do pretty well. Her disabilities so far seem physical. She's very intelligent.
The pp is right that the brain is a miraculous thing. Some people have half their brain removed and recover completely. I'm glad she doesn't want to have an abortion. For one thing, this could be a completely false diagnosis and secondly, the child should really be given a shot in this world. Even if he's not a typical kid, he will be LOVED.
I know a lot of people feel pity for kids with disabilities. Just last night we had a cop over to write up a report because our car was broken into and he asked what her feeding tube was for and when I told him, he said, "oh poor baby". While my DD has been through a lot, I don't pity her. She's a very happy baby and she has a family that loves her. She may not ever walk or talk, but I know for a fact her life is not going to be a bad one. We intend to do right by her and provide her with as many opportunites as possible so she can live her life as fully as she can.
I hope this ramble makes sense. LOL No matter what, your sis is going to love her baby. She will take it all in stride and handle anything that comes her way.