Ronnie is a foil—that is, a character designed to offer a contrast to another character. That character is Roy. Whereas Roy has a childish curiosity, Ronnie has no desire to shake up her reality with thoughts the neighbors might think cuckoo, cray-cray, or verifiably insane.
(Fun fact: Meryl Streep, then a recent drama school grad, auditioned for the role of Ronnie but didn't get it. [Source])
Ronnie doesn't want to think about UFOs or aliens at all, especially the thought that her husband bumped into either on the road. Better they all just focus on what's important in life—Goofy Golf.
When Roy drags the family out to the hill to look for the UFOs, he's clearly flustered, agitated, and a little exhilarated. Ronnie remains aloof. Her response isn't to join Roy or help him figure out what happened. Instead, she tries to distract him:
RONNIE: Don't you think I'm taking this really well? I remember when we used to come to places like this just to look at each other. Snuggle.
Roy decides he wants to keep investigating the strange happening, but Ronnie wants to forget the whole affair and return to normality (read: normal here being a version of working-class American suburbia.) She tries to hide Roy's Two-Face tan with an overnight bronzer, and when Roy shoots that idea down, she comes up with a lie where Roy fell asleep under the sun lamp.
Roy tries to talk to her about how important the UFOs are to him, but Ronnie doesn't want to hear it:
ROY: Ronnie, all I wanna do is, is, is know what's going on.
RONNIE: But nothing's going on. It's just one of those things.
ROY: Which things? Which things?
RONNIE: I don't want to hear about this anymore.
ROY: Ronnie, this is very important. I'm not just gonna let it lay here. I'm gonna call somebody about this...I saw something last night that I can't explain.
RONNIE: I saw something last night I can't explain.
As Roy's obsession leads him to more and more erratic behavior, Ronnie's world is falling apart—their friends stop visiting and Roy loses his job. Her kids are in tears as they watch their dad getting sucked deeper into his obsession. "You're wrecking us!" she tells him.
The breaking point comes when Roy makes a scene around the neighborhood gathering supplies for his giant sculpture. Ronnie looks on with horror while Roy claims he hasn't "felt this good in years." Like a child, Roy doesn't care what the others think of his behavior. Ronnie, terrified, packs up the kids and leaves.
It's easy to judge Ronnie as the uptight, repressed, close-minded person to Roy and Jillian's more free-spirited and open-minded characters. True, maybe if she'd taken time to really listen to Roy, he wouldn't have gone off the rails.
But let's give the lady a break; she didn't experience the encounter, and Roy isn't exactly handling it with grace and detachment. Ronnie's husband's unraveling before her eyes. Under the circumstances, most of us would probably split, too.
We never find out what happens to Ronnie or the kids. They disappear for the entire third act, and we last see Roy leaving with the aliens aboard the mother ship. It's your classic daddy-went-out-for-a-pack-of-cigarettes-and-was-abducted-by-aliens story. Ronnie should definitely keep that family therapy appointment on the books.