My only suggestion is could you talk this through with Chris's family. Explaining your concerns that attending the wedding might be stressful for her and see what they say? They may have already thought of this and have an idea of what to do.
For options of what to do if she doesn't go, I'm not sure. Not had much experience of this myself. Could Chris's family maybe get a carer for one/two days to look after her while they attend the wedding?