Can you help me find the name of this movie?

There is a brother and sister and the little sister dies as a result of the nanny’s negligence, but it looks like an accident. The little girl is standing on the edge of the bathtub, falls and hits her head, and the nanny comes by and doesn’t see er because the curtain is pulled and runs a bath. (or maybe she does it on purpose, can’t remember) but, the little girl drowns. The brother somehow knows it was the nanny and the nanny tries to kill him. The parents dont believe him but the neighbor girl does and tries to help him. It looks like it was set in the 50’s/60’s. My mom and I watched it years and years ago and I forgot the name and she forgets even seeing it. It so creepy she probably blocked it from memory. So. Can you help? Thanksh.