Genetic counselors really and truly, no matter what their education and good intentions, don't know what it is like to be the parent of a child with Down syndrome. I would really suggest she check out some online support groups or speak to a local support group and meet some parents before making a decision.
When my son was diagnosed at birth I was absolutely devastated. Now, 2 years later, I thank God every single day for giving me the privilege of being his mother. He has added so much to our family...my marriage is stronger, and my husband and I have never been closer. Yes, there are challenges, but the rewards are so great. And really, parenting him is not so different than a typical child.
Here are some helpful sites:
People with Down syndrome lead full and meaningful lives, they work, live independently, even drive, get married, pay taxes, contribute to their communities. And even if they need more support...their lives still have great value. If your sister gives her baby a chance, she will not regret it. And if she doesn't think she can handle raising a child with special needs there is a very long waiting list of people who would love the opportunity to give a baby with Down syndrome a loving home.