cod, it is really difficult to understand the headspace of someone who is willing to completely overlook their partner's affair.
it's akin to saying "if my partner hit me i'd leave immediately" but actually when it happens they stay.
maybe your sister has low self esteem and maybe her partner has been doing some low level mental abuse on her?
at this point her entire world and all her expectations for the future have collapsed.
it's a dangerous place to be coz you just hang on and it becomes about winning and not being left.
isn't there some "5 stages" thing that includes disbelief, anger, acceptance etc. sorry, can't remember the whole thing.
it'll take a while for her to get angry, in fact she might never. there's always the possibility that he will sort himself out and they will make a go of it.
most of the women on this thread are dealing with men who's personality has changed and they have had affairs and so on having been previously good people.
is your sister's partner like that or has been dicking (scuse the pun) her around for a while?
i'd echo what dior has said, you can only support her and be there for her. maybe there will be times when you can let rip with your opinions and tell her how you really see it. on the whole tho, until she's ready to listen, you won't be thanked for slagging him off.
maybe she can get some counselling/therapy for herself?
that might help her work out what she actually wants rather than conceding everything to him due to her fear of being left?