I was told at 9 weeks that our baby had possibly trisomy 13 or 18. This was because of high nuchal fluid (what they look for at the 12 weeks scan). We had CVS done and this was ruled out for us. We were told that trisomy 18 is incompatible with life. I am so sorry your sister is going through this. I just don't understand how if they hospital new this why they didn't tell her and organise for more testing, scans and genetic counselling.
There is a charity called HEARTFELT (google it) and they take photos of seriously ill babies and stillborn babies free of charge. Maybe this is something you could organise for your sister?