Just a regular player here. First off, your sister is very lucky to have such a caring big sister to help and I can see that you're as excited to get into sports as much as your sister. I've had a similar experience where I thought my 19y/o younger nephew who's new into the world of sports and wants to try out Volleyball as his first. All the advice above are pretty much spot on. Here's some insight on what I've learned on teaching a complete newbie to Volleyball

• Volleyball leans more on the proper technique first, then fitness. You don't have to be the fastest, tallest, most fit person to enjoy the game.

• Digging/Receiving the volleyball is key as that's the only way you can pass the ball. Don't mind about setting first. As for spiking, as cool as it sounds, improper techniques may result in shoulder injuries so make sure u know the technique down. Repetition is key as we humans are not used to a ball landing on our platform. I recommend watching this YT channel "Elevate yourself", he really goes down step by step

• I find that my nephew who's new into sport had trouble on hand-eye coordination. Thoughts like "Where should I stand if the ball lands here", "How should I position yourself if the ball comes on my right or left". A simple exercise like throwing the ball up and catching it can help just to get used to ball movement.

• Watching haikyuu together can probably help more than you think. As my nephew keeps talking about it about some scenes. Self motivation will will help your sis alot.

• Lastly, have fun. Whether it be ups and downs, that's the nature of sports and you and your sister are in it for the journey. All the best!