Omg, I love you for this! xD
Alright, spoilers *ahem*
This is how I envision it, but it is totally up to your interpretation.
In my mind, Alice and Joshua succeed in finding Jonathan and taking their revenge on him but they make the exact same mistakes Victor and Ethel made. They are just as consumed by their thirst for revenge, stuff happens, just like in Victor's case, they forget they can't control everything and everyone around them, unexpected people show up and much like Victor and Ethel, they find themselves in the vicious, addictive cycle of hatred, revenge and blood.
Not a very uplifting thought, I know, but they chose the wrong things to learn from Victor and Ethel. They chose the same path, hence they suffer pretty much the same fate. I think the biggest difference would be that Alice would be much colder and calculating than Victor ever was and Joshua would have doubts and questions where Victor and Ethel were always so certain. Joshua would be prone to follow his sister, like he always did, but he would also be constantly thinking of Preshea. Besides, Alice would be more cunning, but Joshua has a very amazing power of observation and an understanding of human nature that borders on the supernatural, so I think it would be a pretty interesting ride.
Damn it, I'm getting the urge to write this xD LOL.