How do you say hello to your sister? Como esta?
I only need to say hello
Hola, ¿cómo estás?. (But that sounds like you have too much interest). I mean, if you say this, you are likely not to have seen the other person for some time and you want to know something about her/his. Also, you can use it although you have seen the other person, for example, two days ago. But, then, you are hoping an answer. For example, "fine, thank you... (and more information), ... but the other day I had an accident." Then I would answer: really?. You start to be interested in the answer.
But, if you say: hola, ¿qué tal?. It is just a greeting. You are not hoping an answer although the other person can answer" fine" but it is only like an formulism. And after that, you can start talking.
It depends on your interest in the answer.
With "¿cómo estás?" you are likely to be hoping a longer answer. But, with "¿qué tal?" you are not hoping a longer answer because the answer uses to being shorter.
Unless you are very distant from your sister, ¿Cómo estás? would be far better.
It could be taken as an insult to use the formal form with someone to whom you have had a close relationship. It could be interpreted as implying you are no longer close.
You've almost got it right.
¿Cómo estás? (informal)
¿Cómo está? (formal)