Hmm... typical things to turn your sister into, would be something that demeans her. Probably something to sit on, or something to walk over. A chair, a rug, a beanbag, something like that.
Most likely nothing sexual.
If there was a younger brother/sister available, probably also toys for them to play with, unaware that it is the big sister.
Lastly, something, or anything really, that decorates the room. A pet in a cage or terrarium, a poster on the wall, a lamp, anything that gives her a good view of her room, while he does things with her friends / partners. Personally, I'd prefer if it is not a bed or sex toy or anything that gets directly involved in things, she is just to watch.

As for the friends, anything goes basically, but preferably something that gets used. Toys, clothing, body parts, condoms, towels, food. Or completely change their personalities, turn her female friend into a buff jock, her hung boyfriend into a friendly twink. Make her lovers think they are his submissive pets. Something like that.

If it gets more extreme, forcing his sister to use their transformed friends would be interesting as well. Tf the friends and make it su, that they only turn back, if they are used. You could turn someone into clothes she has to wear, toys she has to use, or turn someone into her dick and have her cum to turn them back. If her friends then start getting into that stuff and start asking Ron for more, that is a total win!

Anyways, those are just some of my ideas. In fact, anything is good.
Looking forward to what you'll be making

I always love the idea of a rug transformation specially a bear being turned into a bear skin rug or something along those line!

I always love the idea of a rug transformation specially a bear being turned into a bear skin rug or something along those line!