The only reason I suggested maybe not to send everything you know immediately was your uncertainty if this was really OM W, and/or the possibility that he might have access to her account and get it first. Now, if you are sure that the sister IS the sister, that gets around that problem.
If that is the case, why not send all you can by FB, (don't say anything about 24 hours, sounds too weird, I would get suspicious of YOU if someone said that to me.) Then give your email and telephone. BTW, ASK her not to say anything yet, rather than tell her. You can only persuade, it's up to her what she does.
IMO, your next best moves will be greatly impacted by what OM W does, and OM's willingness to lose his wife. It's good to have an idea of what you will do in various scenarios, but once you have more info, the situation, and your actions will become clearer.
It shouldn't be a problem if you go ahead and take half the $ now. She stopped putting her check in there and taking your 1/2 of the funds isn't so weird given that you are separated.
The idea of depositing her 1/2 with a family member, I would get a second opinion about. In the US that would be a conservatorship, which you get by court order when someone is so mentally incapacitated that they cannot handle their money, not for folks who are merely depressed or in an affair/Abraham Hicks fog.
Who knows, maybe you won't have to say anything to her family. Best case scenario, OM dumps your wife when his W finds out and your W comes crawling back to you. I wouldn't see the point in telling family in that situation.
