breaking off contact with family.

[beansbeans!]

Tell her, don't tell her your choice, thats up to you. But I highly doubt someone would change their practices/beliefs/attitudes/life on the basis on what one or two people think about them, regardless of the relationship. More likely her response would be anger, as really its not your place to judge her parenting with an ultimatum. (Im not condoning her behaviour/parenting, at all)

From my situation, l spent years explaining what the issues were, and I was never heard. Instead the response would be anger because I had no right to judge their lifestyles, and I suppose thats true to a certain degree. But it was my choice to remove them from my life, because I knew my life would flourish without them in it.

If you were to talk to her, then you need to sit down and have an honest conversation without an ultimatum (you need to change your parenting or we're cutting contact). Yes her children deserve better, but she sounds like she doesn't have the tools needed to handle the situations better. And your younger sister is right, unfortunately it is acceptable (after all there are parenting 'educators' who advocate that sort of parenting). If you aren't ready for the relationship to end, or really don't want it to, then perhaps you could gently guide her into a different parenting path? But in the end the way we all parent is our own choice.

[myturn]

I'm pretty confident that child protection would investigate the use of an implement to smack a child. It might not 'go anywhere' further. But if it's not reported now then it could escalate to other things in the future and there is no record of this.
If you are in Victoria they also have an organisation called child first that DHS then refers to which us designed to support people make healthy parenting choices. And teach them if necessary.
I work with children and have reported things like this just to make sure there is something on file as a minimum. But I am pretty sure that using an object to smack is not okay. And would be investigated to start with. Either way you can call and consult with them without using names to start if you were uncomfortable.

[BlackCat]

I'm fairly certain that smacking with an open hand, lightly so as not to leave a mark is acceptable. Smacking with an implement, leaving a mark or hitting above the neck is not acceptable and may constitute further investigation. It wouldn't hurt to ring and ask as obviously you are concerned.

My own family parent quite differently to how I do. I chose not to break of contact with my siblings over their parenting choices as I would rather be in my nieces and nephews lives. However, I took a big step back. I can handle her parenting style in small doses and I feel reassured that the children are ok from seeing them and we still have a relationship.