In my experience it can take years of a relationship to get to know whether you want to do something like getting married.
I couldn't agree more! I've never understood this, "We're soul mates and madly in love" after a few dates crap. Yeah I can see you really hitting it off with someone and loving spending time with each other...but to me, that's the easy part--finding someone to get along with. The hard part is finding someone you get along with and wouldn't mind waking up next to for the next 40 odd years.
I just don't want to see my sister get further entrenched in the borg only because she is lonely and horny.
Sadly--this is what happens to most JW's. It sounds like she has had some really crappy relationships. It also sounds like she is on the rebound. It happens.
I have been in quite a few situations like this...sadly. You can talk till you're blue in the face, and it usually doesn't matter because they are "in love" and you just "don't understand...it's different this time".
It sounds like grandma may have been misinformed tho...and perhaps your sis was right, and grandma misunderstood. One can always hope. In the mean time, just be there for your sister. Voice your concerns, but eventually the choice is hers.
(However in a pinch might I suggest chains and a combination lock only you know the combination to? Hee hee!