cod, sorry it's come to this but better sooner than later.
you do sound like a great sis and i hope the holiday and the family rallying around will help your sis get thru this.
if bil is an okay guy as you have said, appeal to his better nature and ask him to stick to what he has said.
your sis will be confused enough without him popping in and out as he chooses.
before long the mental breakdown will probably become used as an excuse for the affair. your sis will be inclined to support him thru his mental health issues. your bil will be having his cake and eating it with cream and ice-cream and a cherry on top.
he might not even intend to do it, but he will turn to her and she will support him.
and they'll end up in the land of limbo that i occupy 3yrs down the line.
clean break, i wish i had done so.