First of all its really good that you had that heart to heart with your sister. I think it meant alot to her.
It definately sounds like there was a cycle. It sounds like as soon as you realised it wasnt an appropriate thing to do you stopped, perhaps your brother stopped when he realised. So your older sister was the first sibling to start the sexual games but i am curious to know who taught it to her.
I think your parents reaction is too laid back. If i were the parent i would want to know where it stemmed from, ie was your elder sister abused.
Im guessing that the guilt you have towards your sister, your older siblings have towars you.
You were only a child and following learnt behaviour. It seems like you stopped when you undertood it was wrong.
I am so sorry for you and your siblings.
Have you thought about speaking to your older sister to ask her if she was abused?