Why were the sister and her husband so rich?

Maybe they weren't rich. Maybe they lived beyond their means so as to give the impression of wealth. I know people who practically bankrupt themselves just for the privilege of being able to say that they live in [insert desirable area]

I don't know but their storyline was annoying and I could've done without it or at least taken a smaller dose of it.

In the sequel, "This is 40", they focus on Pete owning a record label.

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Pulp can move, baby!

This is Hollywood. All characters are either upper middle class or Black (working class)

1. Her husband is a record producer. Major $$$ there.

2. It's a movie. Anything's possible.

The husband is a record producer. In Los Angeles, a major music industry hub.
Actually their home was relatively modest compared to the way some people live who are in the L.A music and film industries.