Hi. Here's my opinion, but just so you know, I was a JW for about 20 years. Now I'm an atheist.
You say your sis has a history of bad relationships. guess what? She just got into another one!
While "technically" these two are not witnesses, they are already behaving as witnesses. they are practicing their lying and hypocrisy, and have already discovered that it only counts if you get caught. They are in the process of entering a morally bankrupt cult.
By the way, JWs will tell you they ARE christian, so when your sis says she "still is" that doesn't mean anything.
The harsh reality is that until the time that your sis and her boy toy get tired of the abuse they're about to receive, there is no way that you can do anything to convince them of their error. They have been taught that YOU, as a non-witness, are a servant ot SATAN and are bound and determined to see them perish when Armageddon comes. Armageddon was going to come in 1975, but it's running a bit late...
It seems that you care for these two very much. for now though, there is nothing you can do except save yourself. by that i do *not* mean Jesus. I mean get an education, be physically fit and emotionally well balanced. Enjoy your time on earth, and do whatever you can do.
best wishes. it can be a harsh world sometimes.