My whole family is getting my sister's missed calls

Hi, this is my first time asking a question so I hope someone can help me!

There are four iPhone 6 owners in my family (me, mom, dad, sister). My sister and I have updated to iOS10, my parents have not.

Previously, this had only been happening to my mom. When someone called my sister's phone, it turned up as a missed call on my mom's phone and it never rang to my sister's phone (so essentially no one's phone rang, my mom got the missed call notification). We never could figure out why it was happening, so they just ignored it. Now it's happening on my phone, and I want to make it stop. It's super frustrating.

Another weird thing is that if I call my sister's phone, sometimes I get missed calls from myself, and my mom gets a missed call from me.

This is the same for Facetime calls and for phone calls, BUT IT DOES NOT HAPPEN WITH IMESSAGE.

We all have the same iCloud account, but each of us have a different one signed in for iMessaging and Facetime (except for my parents, they share the originally iCloud for iMessage and Facetime).

TL;DR: My sister's phone doesn't ring, we get the missed calls from it, really annoying, not sure how to make it stop.

Thank you in advance!!!