Yeah, it bothers me that my fWH has continued to have new partners after marriage; I'm still at 2 for my lifetime! But then again, that's likely one factor that made it easier for him to cheat--he did not tie sex and love together.
No slam on your sister, of course, but just because a couple has been together for 40 years does not mean there has been no infidelity. Lots of couples have been through this and stayed together; many are better than ever afterwards.
That's what we are all going for, right? My H and I make a very loving couple; sure does not mean no cheating has occurred!!