Mi mom get married since 3 years ago and now that I know my stepsister I love her and she loves me but we don't know how to tell it to the other one and to our parents

Do yourself a favor and DON'T say anything... find someone that doesn't share a parent with you in any way shape or form. That's MOST of the worlds population.

Before I give any advice on this I need to know how old are you and your step sister?

Please provide some additional details. Are you a teenager?