I am sorry to hear your sister is going through this. I have terminated two pregnancies for fatal fetal abnormalities,and found much comfort and understanding on the thread here entitled 'women who have chosen to terminate...' (sorry I don't have the link to hand). I agree that your sister would find comfort and reassurance here.
Since the two terminations, I have had a healthy baby. Your sister is far from too old to have a healthy baby, and T21 is mostly a random occurrence, and thus v unlikely to recur.
In terms of how you can support her at the moment, as she faces the termination and the time afterwards, then I think being there to listen to her/reply to her emails (however she wants to communicate), but also following her cues, so not fussing too much over her and allowing her to set the tone for what you talk about (she might want distraction as much as comfort) and how you refer to the baby (some women feel very strongly that they would like their families to acknowledge the baby as a person they have lost - others very much want to think of it as a foetus rather than a person - try to follow her lead on this). If she is worried about the process, it might also be worth her posting here as lots of us have been through it at similar stages of gestation.
In response to the earlier debate, yes, of course amnio doesn't show whether it is severe or not BUT the scan that presumably triggered the amnio can - ie heart problems etc can be seen then. The amnio may have 'explained' the severe problems already picked up on the scan.
I feel very strongly for your sister, as I think it is all the harder when this is her first baby and she has had a struggle to conceive.