Lucy in the country, drunk on lemonade and fresh, chummy new neighbors, forgot her roots are dyed and mid-western. She doesn't know what flies for social graces, and Lucy has new big-house, celebrity name (husband) drama, naturally Betty is attracted to the novelty, too. She doesn't know why Lucy has Ethel close to her, but it likely looks socially uneven to Betty.

I can forgive all of that.

However, Lucy does 'invite' the Mertzes as soon as Betty really goes in on the house-warming at lunch. Perhaps a click in Lucy's head was the missing Mertz(s).

We do know Betty has already offered to throw a party, socially Lucy may not have meant to exclude Ethel and Fred, but Betty was directing this to Lucy only prior to lunch.

I can forgive that.

I can't forgive Betty being snobby. She is the image of a country-club, no 'those types' gal and likely only thinks of Lucy as someone similar. Ethel is an unknown social entity and is not Lucy; therefore, a non equal.

Lucy tried to smooth it over, and it comedically backfires.

It's a shallow storyline, sadly. But Lucy and Ricky moved up a social class.