Chinese in laws visa questions (green card, uscis, application, migrate)

[Mattks]

My wife is Chinese and has a green card, she's been here 2 years now on green card status, before that was 4 years h1b, and 2 years student visa. Her parents are Chinese citizens and have been to the US twice in the last 3-4 years, no visa problems, they stayed about 6 months each time on regular tourist visas. We would like to get them a longer visa, but I am unsure about what visa for them to apply for. Also, I don't think we can at this point, but how would sponsorship work? We did all the documentation and applied for the green cards ourselves, so I do have knowledge on how the immigration process works, but I don't have as much time to research as I did in the past.
I wasn't sure if I could sponsor her parents under this since they are not my parents, but rather my parents-in-law. Or if we have to wait until my wife is a citizen before we can do anything.
Green Card for an Immediate Relative of a U.S. Citizen | USCIS

I've read the the US is reluctant to grant green cards or other forms of permanent residency to older people who may have health problems and/or financial problems, and could become reliant on government for assistance. They are fine in both regards, I'm just posting this so people know a little background information, they shouldn't get rejected on these regards.

[Threestep2]

Travel.state.gov walks you through US visas from soup to nuts.
B2 can be extended by up to 180 days in country. Read through the application. Investment is an option. Otherwise your wife can sponsor them as soon as she has her US citizenship. But are they ready and willing to leave China?

[Mattks]

Thanks for the reply and the link, they don't want to immigrate from China, but they would like to be able to stay longer in the US when visiting. Me and my wife own a business here and they help out quite a bit, they are part owners of the business, but it's not enough of an investment for that type of visa. Sorry, I don't know the names of the different types of visas, but I believe the minimum investment is $250,000 for that type of visa and I don't think they are willing to invest that much since they are retired and living off savings. I was searching for the investment type visa and didn't see that on the travel.state.gov website.

After reading the link they have applied for B2 in the past, even though technically they should probably apply for B1/B2. I'll look into that 180 day extension, I didn't know about that.