It's your choice. BUT if they are behaving this way at your house chances are this is what they do at home. A few questions you can ask yourself or things to consider:
Do the parents seem willing to work with you on any behavior issues? (if not? let them go)
Do the boys have respect for you and your authority now? (if not? let them go)
Is this behavior affecting the other kids or your abilty to care for the others? (if yes, let them go)
Is their behavior violent or threatening toward anyone? self,each other,others. (if so, let them go)
depending on how old they are, you may be able to turn things around (the parents could be extremely grateful for your abilities) BUT....consider the behavior as something they are very used to, if you think this is how Mom and Dad allow them to behave at home, it may be a loosing battle for you. Consider the use of your termination period and use it if you have to.

I had a pair of brothers that came to me a few years ago, they fought like cats and dogs with each other, and hit, scratched, kicked and bit each other. That to me, is not normal brother behavior...it seemed very violent and after watching their behavior for a few days, came to the conclusion (due to the responses they gave me when I tried to redirect or discipline) they did this at home and got away with it....it tried for a week. Then I got a call from a dif Mother that her daughter had been bit in the arm..... I knew who the cullprate was and was not willing to open my home to harm to other children due to the fact that these kids parents had no control over their sons. serious anger issues. Like I said def up to you, but consider if it's something you can control while they are in your home...if not, let them go!