I'm talking about the main four. Who people usually think of when they say the Fantastic Four. Frank and Val usually come up after that initial thought. Also, I don't think even Johnny fits into the nuclear family if you only go by the Richards. Uncles, Aunts, and the like are often not included in the description. Having to remove Ben or Johnny to fit the description tells me that the FF aren't a nuclear family, they just aren't complete without them.
I think nuclear only applies if you look at Reed and Sue with their kids. I'm talking strictly about the main 4 from the beginning they weren't a tradition nuclear family in my opinion. It's something that has been ascribed to them later on to make them seem more dated than they are.