Thank you so much for your advice. I took it and after an initial conflict things have smoothed down.

This is also to provide a general update. I talked to my sister about age appropriate behaviour. We have decided that for now we will dial down the physical affection and she will also tone down her childish manners. But we have also agreed that the whole dynamic can't change overnight. It will be a work in progress and I'll need to be vigilant that we don't slide back into our familiar roles. Thank you (all of you) for your solid support. Sharing my problem with you was a very good idea and I am really happy we had this discussion. It helped clarify a lot of things. Sending you loads of good vibes. Stay safe and healthy.