Blended families are not easy huh?
But truth be told, having been married twice myself, I have to say....
Unless the children are being mistreated, neglected, or abused, MOST of the time it's better to just stay out of it...
I know, that seems so wrong to me too, you're part of this family too....your husband should value your opinion...and you certainly need to voice that while he and his ex are in charge of their children that you have very different views and will not be parenting your own child that way.
But when it's all said and done, they are their kids, and they have the right to screw them up all they want.
Now...fast forward until your child is older....
If step brothers/sisters become a danger or bad influence over your own child your role changes, because protecting your child is your primary concern.
Try to just be calm, stand back, and let them make this mistake. We probably all think it's a mistake to allow children to be the boss, homework and education should be the number one priority, soccer and other activities ... like television or video games should only come after school work is finished.
Hang in there !