My spidey senses are tingling, and I'm not liking this dynamic with the sister. This won't end well.
Like I said: "And then it just happened..."
PiGA, from this point on you must assume that there is at least an emotional affair going on between your husband and your sister, if not a physical one. I would refrain from snooping unless you're sure you can deal with finding proof of an EA or PA, but keep your eyes and ears open.
I believe Puppy says that it is very rare that a man leaves his wife just because they're not in love anymore; there is almost always another woman involved.
And I am. The problem is that I need her. I am so sad over this. My kids LOVE her kids and it would be good for them to spend more time with them. It is very good for my daughter. My sister is very adamant that nothing is happening and I still find it hard to think of them together. But, I never saw myself getting divorced either.
You "need" the woman who is having -- AT BEST -- an inappropriate relationship with your husband, and you think "it would be good" for your kids to spend more time with the woman who is likely contributing to the breakup of your family?????