I hate to sound so negative, but how exactly do they get into foster care? I mean the only adult person they knew was evil and extremely powerful. Wouldn't it make sense they'd avoid grown ups and people altogether? They'd probably strike out at anyone trying to detain them. A person taking them in would firs have to gain their trust or at least interest (food, puppy, playmates) but from there to foster care would be abandoning them again... Unless the person gaining their trust turns into their foster parent - too good to be true? How they come into 'normal society' would have a big effect on their reaction and development. And with their history, they won't be forced in. This also throws up the question of how they will react to being disciplined - do they laugh at being grounded (because it's such a mild punishment) or do they throw a fit and blow up the roof? If they didn't kill the grandmother themselves, but were rescued - that might make their transition smoother and a fair bit easier to explain...
So how could they end up in foster care? Let's assume a few days after their Gran's death, they just left her estate and wandered into the city.
Motivation This sounds a lot like the Fritzl kids with magical powers. A lifetime in a basement meant they developed their own language, and of course their knowledge of the real world was very limited. Add to that dark magic and you've got the potential for quite a confronting story. You story can go in various directions depending on how powerful they are. If they're only powerful enough to impact on the small world around (tricks and illusions and the like) them then you might be able to keep it tight and psychological. If we're talking about something more powerful (as a sinmple example the massive mind control powers of professor X for example) without the ehtical constraints of a proper upbringing then you've potentially got a wider story, maybe even on a geopolitical scale. The most important thing in any character is motivation. What do they really want? Do they crave the affection that they have never had (ie are the "good") or are they after emotional retribution like Anakin Skywalker - "anger leads to hate, hate leads to suffering". I like the overall idea, it's my kind of dark, but maybe think about their motivation.
Their powers are pretty large scale, Marvok can make use of wind, water, wood and spirit; which is your basic otherworldly, psychic magic. Edelyn can use fire, metal, lightning and earth. So yeah they're pretty powerful. Does that have any particular psychological implications?
Yes, it would seem so. Exactly, no two people ever respond the same way to the same events, there is comon responses, but differing views on the subject. They would at least have the basic understanding of love and affection, rather then the world being totally negitive. Definately a good question, since they would not trust anyone. Grandmother probably told them the world was worse then she treated them to keep them from running away. Great point on them feeling abandoned again. If they latched on to the first person that comes along(bonding with everyone) they would be traumatized when that person leaves them. Fear of the new person, possibly being just like their grnadmother. If they survive, they would not see the need to find a "good home". If they scraped by they would still be fearful of anyone but might be more accepting if they have gone hungry or suffered during that month. They might even need hospitalization(mental or health problems) or worse one needed it and the other didn't, seperating them would not be healthy for the person trying to do it. Its the two of the against the world, seperating would mean they are being divided to be conquered. ie paranoia could be a problem. Like someone else said, you decide, we are just offering suggestions, and also look up the problems yourself. There might be the perfect problems for your story in comon adoptive conditions other then the ones I mentioned.
It's not that he isn't listening. Sometimes it's easier to develop characters if you can talk things out. That being said I think you need to jump into the characters heads a bit more. Background is a great tool but you also have to be able to answer the questions your posing in some degree to be able to filter the information your wanting on these conditions. What I mean by that is perhaps when you ask about psychological conditions and how environment can change behaviors throw in something your character might think or say/do.. it might go a longer way to helping you flesh them out more.
^All Dread is doing is getting us to make his characters. He isn't thinking about it at all. Or at least THAT'S what it looks like. Maybe if he wrote his own ideas with ours I'd be more inclined to go easy on him.
Have you ever seen Black Lagoon? The story you told reminded me of 'Hansel and Gretel', twins that were abused into madness and who just go ax-crazy. I suppose you could go that route, but they're going to be a bit strange to say the least. Lots of mental issues, maybe they only trust eachother to the exclusion of the rest of the world. It depends where you want to take them, either working out and dealing with their problems, or dealing with them slightly so that they can cope with life though they might have a bit odd behaviour, or completely go off the rails.