I grew up learning cantonese and then english (im better in english than cantonese). I want to learn mandarin instead so how long will it take me to learn mandarin if i already know cantonese? Plus my parents and sister know mandarin so we might try to speak to eachother in mandarin.

I learn mandarin from a site called Chinese-tools.com so if you know a better website to learn chinese please tell me.

If you mean learning to speak Mandarin, it won't take you long, maybe 6 months if you practise frequently enough. One of the biggest challenges in learning Mandarin is the sentence structure. But if you can speak Cantonese, the problem shouldn't exit for you.

You Cantonese vocabulary is the basis of your Mandarin one. The more you already know, the easier you learn Mandarin.

But for writing or reading, it will be tough if you don't know any before.