Susan - "Look, Diane, we need to talk. You're supposed to mull these things over for literal years of actual time. You can't just figure something out, be immediately confident about it, and start actually talking to other people about it! That's not how these things work!"
Diane - "...Y'all need me to smack you upside your heads, don't you?"
For the record, Diane keeps tons of secrets from her family. It's mostly about stuff like how many cookies she's actually eaten and when she actually got home some nights, however.
Also, how much she actually cares about Rhea, because that's how siblings work. Healthy siblings, anyway. You get some weirdo siblings who will be all open and honest about the importance of family and how much they mean to each other, but that's CLEARLY abnormal weirdo nonsense right there.