If someone has Alzheimer's is it ok to pretend I am their sibling, rather than their forgotten child ?

While it is not okay to keep on correcting them, or tell them they are wrong for calling you their sibling, it is in fact important to remind them who you are and stay present regardless if they still seem confused. Their acceptance of your presence is a good sign. Even if they were to remember that you are their child, it does not mean they will not forget that in a couple minutes. People with Alzheimers often react well when a person is able to simply have a casual conversation with them, and make themselves available to whatever they want to talk about.