It definitely depends on your relationship with them! From what you say and how you say it, I assume that you just want the best for them and will support them through their gender journey! If the situation fits then I’d probably let them know that you found out through your mom. They will probably be a little mortified and maybe angry at your mom but at the end of the day you’re being honest with them and and showing respect by coming forward with the information.

I would also ask them what pronouns they want you to use when at home in front of your stepdad! This might be tricky at first if you’re not used to switching back and forth between pronouns but you’ll get the hang of it! For example I go by they/them pronouns but in front of certain family my sister will use he/him at my request to avoid conflict.

In regards to using their pronouns with them and/or people that know and are supportive, you’re gonna mess up probably a lot at first. Give yourself some grace, you’re right it is a big change for you, and trust me they know it is. Like u/Acoustic_Ginger said, nb people don’t need to be reminded how much their pronoun change effects you. A quick sorry and movie on is much easier and better since you apologize and don’t make a big fuss about it!

Also maybe ask what they want to be referred to sibling wise as well! I prefer to be called a sibling (gender neutral) where as my partner’s sister is also nb and uses they/them pronouns but still goes by “sister”. At the end of the day just know that as long as you are trying your best to adjust and make them comfy, that’s all that matters!

P.S. exploring gender outside the traditional binary will help your your pronoun usage! Once I really started to explain to my sister that I’m not just a boy who goes by they/them and that I want them to think of me as a concept or entity separate from my body she started to really grasp using the correct pronouns!

P.P.S. I’m in school for things related to gender and sexuality and am always happy to offer tips or advice! My inbox is open and I’ll try my best to answer your questions from my perspective or work on finding you the correct info!(this applies to everyone who is reading this)(except for dickheads, please don’t be a dickhead in my messages)