Contact details get edited after a message

Hi,

I have my Contact Book sorted by surname.

For family, I just input the first name - eg. Carrie will be listed under C with Mike Connelly.

However, every now and again I find that three of my sister would email, text or iMessage me and my Contact Book would then list them by their surname.*

They all use gmail, but I don't think this is the cause - only one of my sisrer's contact details changes every time.

Anybody know what might cause this?

Surely the name I choose would overide any other.

Cheers.

* Actually I can't be sure that it is when they contact me or I respond.