Could me coming out negatively effect my younger siblings? they know other kids with "two moms/dads" so this whole thing is not unheard of to them. At ages 6 and 9, is it inappropriate? should I wait until they are older? Should I tell my family but keep them out of the loop until they are older?
If you think your parents will respond well, when you come out to them they can help you figure out how to address it with your siblings
I second the above poster, talk to your parents about it first and see what they would prefer you to do.
I agree with talking to your parents first. At those ages, your siblings probably aren't fully aware of what sexuality means, and even then coming out would be more informational for them than positive/negative.