It’s paternal, but feel free to sit down.
Fraternal is brotherly. Anyone using the term “fraternal grandparents” is either covering a very unusual special case, or is making a mistake. Evidently it’s a common mistake.
If a mistake becomes common enough, it gets documented in dictionaries, and people assume that means it’s correct. Eventually, all words will be synonyms. Won’t life be simpler then? We’ll never say the wrong thing.
snerk snerk
Just out of curiosity, I ran a Google Fight and got: paternal grandfather - 568,000, fraternal grandfather - 29,000. So like everyone suspected, it’s wrong, but it’s pretty common. They also sound similar enough to mis-hear.
I ran across something similar with the non-word: centrifical.
I saw that movie.
Malkovich malkovich. Malkovich? Malkovich!!
The only reasonable use I can come up with is if it was the grandparent of your (step)-sibling “on the other side”. But in that case I would have referred to “my sibling’s [side-appropriate] grandparent”, and I hope the coding makes sense.
Sister rapers. Brother stabbers. Brother rapers!
You said rape twice.
Hodor.
It would probably be difficult to get sorority girls to come to your parties.
Just throwing this out:
A fraternal grandparent could be your adopted sib’s biological grandparents.
That’s true. I learned something today. 
QUOTE=WhyNot;16897410]I don’t think people should be too hard on you. It does make a small amount of sense, in that fraternal has something to do with male, and words mean what we hear people around us using them to mean. You’re certainly not the only one who’s ever made that error. But now you know. As the cliché goes, you’ve learned something today; must be a good day. 
[/QUOTE]
Were you on the Group W bench?
“What’d ya get, Kid?”
“I didn’t get nothin’. I had to pay $25 and pick up all the garbage.”
Er, no, “fraternal” means brotherly (in its literal and metaphorical senses), and not anything mean remotely related to “lecherous”. If it did, it would be alluding to lust for your brother, which is sick! “Fraternizing” does not mean anything to do with sex or lechery either. It means hanging out (with), and being friendly (like with a brother). Sometimes fraternizing with people of you r sex of attraction does lead to sex, which is one reason why it is discouraged in certain circumstances, but fraternizing itself has nothing to do with sex or lust.
FWIW, there are businesses that use “fraternization” between managers and non-managers as a euphemism for sex. It was in the code of conduct at the bank I used to work at, IIRC.
Which might well have eventually happened to Rolling Stones bassist Bill Wyman, when his child bride’s mother got romantically involved with his (Wyman’s) adult son.
IF I’m remembering correctly.
Most defiantly.
Except for the ones from the maternity houses.
Franternizing (AFAIK) usually means “getting familiar with”, not in the family way, so to speak, but socializing and becoming friends. Fraternizing with the troops (or the locals) is frowned upon because familiarity destroys the effectiveness - less likely to give difficult orders or follow them, etc. I assume the bank used the term so they did not have to prove actual penentration to fire the couple (or, based on logic from decades ago, the woman). Dating was sufficient.