project-danny answered your question: Headcanon Time!
hmm.. I believe as Danny gets older, by the age of 17, he is at the exact level as Dani, though she was probably stronger than him before..?
... Yeah, I could see that. It really depends on what they end up doing, and which powers they use the most. Danny's ecto-blast will probably always be stronger than Danielle's, for instance, and I'm not sure if/when she'll ever learn to use the ghostly wail. But more defensive stuff, and the powers less useful for fighting head-on, I think she'll be better at.
run-awaydannyphantom answered your question: Headcanon Time!
Reluctant agreed. I like to think that Dannys the strongest, but I think that Danielle could eventually surpass him.
Haha. I think most everyone would assume Danny's the strongest, possibly including Danielle. She's just easy to underestimate. But every time we see her in the show, she's impressively competent with her powers. For instance, does that rapid fire attack, using hands and feet, which I don't think we see from anyone else, ever. I could be forgetting something, but that scene really sticks out for me as to how powerful she actually is.