So far:
Round 1: Elsa

I don't think she isn't strong- just, the other princesses are stronger.

i guess aurora didn't showed (or didn't get the change) to be strong

Her.

Aurora is strong. It took an active choice to do what "duty" made her do... Leave her chance at love and romance behind in the woods, and return home to a foreign place where "supposedly" you're now a princess. I consider her collectively strong in her own way, and certainly just as strong as Snow White and Cinderella.

Yep.

I didn't mean to put the word not! Please ignore that!

Compared to the others she's weaker.

I think Aurora is one of the strongest princesses emotionally as it is harder for someone to control their emotions than to just let them control you. Ariel is an example of a princess to me who is not emotionally strong at all. She doesn't control her emotions, her emotions control her. She is very impulsive, flighty and easily manipulated by Ursula as a result of her emotional vulnerability. She doesn't do anything to prove she's physically strong either.

Also, Aurora is very selfless with her emotional strength. She doesn't believe in causing a scene and creating extra burdens on everyone with emotional tantrums just because she didn't get her way. It takes a TON of strength to look beyond oneself and do what is best for everyone else as a collective instead of only thinking of oneself.

She gives in pretty easily, even on her own morals. Or Ariel.

To marry a prince because you were betrothed to him while you were in your nappies was expected of princesses of that era, but I honestly, don't see that as healthy practice. People were considered to be virtuous without ever examining their actions. How would King Stefan would know Prince Philip grow up to be a brave and honorable prince to be. To follow blindly into what was expected out of Aurora, without once voicing your opinions and discussing it looks weaker to me than Ariel's calm way of defending her personal choices and rights. She became emotionally vulnerable in a moment.
Yes, Aurora showed strength in respecting her aunt's decisions as they had raised her all her life and was her well-wishers, but I'd have preferred if Aurora would have at least shared her concerns once and taken an informed decision.

Same that I told in the previous poll, she's too weak and kind of damsel in distress type.