Didn't think I'd need Seating Assignments

How many guests are you expecting? Maybe you could assign your father and his family to one table and your mother and her family to another? Not necessarily assign seats, but just tell them via word of mouth that you'd like them at separate tables.

If you're making a reservation for a group, you could show up a few minutes earlier and put out place cards down at the reserved tables.

I think people could probably figure that out, but you could definitely tell them ahead of time that you're expecting to sit at table A, have your mom at table B, and your dad at table C to ensure the right people aren't put together, and then let everyone else fill in as needed.

Hey there!

As Caytlyn mentioned, if you want to avoid using place cards, perhaps you could verbally tell those 4 individual which table they are to sit at and everyone else can choose their own seat?

If you don't want your mom and grandma to know that your dad and step mom have requested to not sit with them (which may cause some hurt feelings), creating place cards for everyone may be the best option. You could do this at home by printing them yourself. They don't have to be fancy, a name on a piece of card is sufficient!

How many guests will there be at the dinner in total?

Tell your dad and step mom that they can show up early and choose their seats and they can hope for the best.

They are grown ups, you shouldn't have to worry about it, this is your wedding day

I'm not too worried that my mom will know my dad and step mom hate her. I think after 25 years that's already well established HA! I just don't want my mom throwing a fit or purposefully trying to annoy my dad. I actually haven't seen her and him in the same room since I graduated High School in 2004 and even then they sat on opposite sides of the gym.

I think the getting there early to put down place cards would be best. We recently had to go a more traditional route with food anyway so they'll probably not make a big deal out of us showing up early to assign seats.

Usually you can send a seating chart to the restaurant and they will put out placecards as long as you plan it beforehand.

I wouldn't bother yourself with seating assignments. If he doesn't want to sit with them they can be sure to arrive early enough to pick seats away from them.