Need some not-horror custom story to play

[Ashtoreth]

Unfortunately I can't think of any custom story that matches your requirements (I haven't played many Amnesia mods, to be honest), but Platform Perils might be a good one for you and your sister to play, considering her age. It's a short yet quite entertaining platforming custom story which doesn't contain any monsters or scares, basically it's just timed jumping. You could give it a try.