Sorry to hear you have such a tsk ahead of you. I think it is probably best to simply do the usual, please sit down, we have some bad news, opening. That is assuming the older sister is capable of taking in the news, but you seem to suggest her mental facilities are fine. Was that not the case there would be an argument for not upsetting her be volunteering unfortunate news. I know someone who reminded a relation that her son had died years ago and they didn't take it well at all, would have been much better to not remind them and let them go on thinking the son hadn't visited for a while. Unfortunate timing though if she is already feeling down. Best of luck to you.