Not with the length, but the content. I think the game for Persona 5 is rated M, and coming from someone who played a little of it I wouldn't show it to a little sibling.
Don't start with anything that is extremely long. keep it short. save the cringe shit for when she is actually really into anime. For action i would start with something like Black rock shooter. I also highly recommend Toradora. If you want something goofy then i highly recommend Lucky star and Azumanga daioh. An honorable mention is Metropolis 2001. I also highly recommend any studio Ghibli movies.
i would suggest "by the grace of the gods". its the most wholesome anime i know of, and as someone who doesnt get emotional too often i can tell you this one made me feel the warmest of fuzzies
One of my first ones and my personal favorite is Konosuba, it's a great comedy Isekai. There may be a slightly weird scene or two once in a while, but for the most part it's kid-friendly
My best recommendation is Lord of Mysteries, which is an isekai but different from more conventional ones. It’s British so grumpy British swear words, but I think they’re rather infrequent. There isn’t any nudity, though episode 9 is a bit suggestive for the first half, and it isn’t too violent. The biggest thing is some disturbing imagery
