I'm going to ask my sister not to open new credit card accounts
You have a nice idea to help out your sister's finances. Does she spend wisely?
I think she is too impulsive, at least more than me.
If you can help her out then you should, she will be grateful for you later in life. Be direct and not overbearing and things will be okay between you two.
I don't want her to suffer from the financial aftermath.
You are to be commended for your kind act of caring for your sister's finances. You can easily make her lots of money this way.