@Aurora: I think most Pixelfed instances/apps have a way to block NSFW content. You'll have to make sure your sister has NSFW content blocking on (and keeps it on) and you'll want to check in with her every now and then about her experience there. I think marking images as NSFW is optional for the person posting the content, so not every single NSFW image will necessarily be blocked. It's hard to give someone a completely clean social media experience.
One other thing you can try: look for instances that are focused on a topic your sister enjoys that is not likely to have NSFW content in it, like ppl who are interested in trains or playing a musical instrument. There are some here: https://fediverse.party/en/portal/servers/ (but you should check the server before she joins)
to all the commenters saying the fediverse is not a good place: these kids are going to be on social media, one way or another. It could be tiktok, bluesky, snapchat, or whatever else they can get onto. Some (but not all) parts of the fediverse should be accessible to them. Ofc that means someone has to do the hard work of hosting those instances, which is not easy. Just saying "this isn't for you" isn't doing teens or the fediverse or parents any favors.