how do you deal with a selfish daughter/son?

I am not a child expert, but I do know that children learn by example. I would show them how bad it can feel for someone to be really selfish toward them. I lived with my sister for a while and she has six kids. You can't buy for one child and leave the others out, it's just not fair. But in your case, you may have to do this for a little while to show these two how to share. They need to understand that everyone has to share with someone. Maybe, you might buy a special treat for the others and leave them out a couple of times to let them know that life is all about sharing. Make them realize that everytimes someone gives them something they are sharing and if no one shared with them, they would have nothing.

i agree with rozie, i guess you have to leave the 2 out for a while. but are you sure they are really selfish or jealous of the 4 or maybe jealous of just one sibling of theirs. i guess you must really talk with your children whom you consider selfish. they might just lack of affection from you or too much of affection might cause it? im not really a relationship expert myself but communicating with them is the best thing to do in a family situation.