Messages from mom and dad only appear in my moms messsge logs

So I have an issue!

I have 2 contacts: Dad Mobile & Mom Mobile. I can text them separately but as of late when IOS12 came out, I have been running into an issue.

When I go to text my dad, I’ll input his name into the new message contact field, click it and it will bring up my moms messages. Then when I message my dad, my mom will get the the message instead!!!

I have tried deleting the contact and making a new one but it still has the same effect.

Can anyone help me please, this is annoying and it is bugging our whole family because my mom recieves messsges that my dad should only get