Is it okay to tell my parents how to raise their kid
why do you want to tell your parents how to raise kids?
because I think they need to know about it.
You're talking as if you've more experience raising kids than your parents. You have no right to tell them if you're parents are doing their job right or wrong.
I should because I know those little shits better than my parents.
If you know things about your younger siblings that they are hiding from your parents maybe you could mention it to your parents. There is no need to criticize their parenting style in order to do this. They'll be more likely to believe there's a problem if they don't feel attacked.