iMessage issue between iPhone 6 & iPhone 5s

My iPhone 6 cannot send/receive iMessages with my sister's or mom's iPhone 5s. If in a group message with a 6 & a 5s it breaks it up into two messages, one with iMessages and one without - on my sister's phone it shows it all in one message.

When I was in a message with two iPhone 5s's it would not let me send any texts at all in a message my mom had created - as iMessage or text message - until I created a new conversation.

It does send/receive from iPhone 6 & 6s users.

This just started about a week ago. All phones are updated to the latest firmware as of today - iOS 9.1 - and it is still not working.

Help?