Hello I am having a weird calendar problem.

I have a galaxy s10+.
My dad has an iPhone 11.

Everytime I add an event on my phone calendar...the exact same event gets added to my dad's calendar. I looked at his calendar history...and every event I entered on my phone is also on his device. It copies the event name and device.

The only commonality in our phones I can think of is that we both control the same nest thermostat. I believe the next is controlled by my Google account.

I do not have an iCloud logged into any device. I have not had an iPhone in 5+ years. he does not have a Samsung or Google account logged into any device.

What the heck is going on?
Can someone help me stop this please?
Thank you.

 

Another thing is my calendar updates are also going to my sister's iPhone who lives with me. But when my dad and sister update their calendar, my phone does not get it.