That means it will be almost 3 years of doing that by December when they return. If they saw the exact same officer who gave them a chance last time, they might be OK, but they'll almost surely see somebody else, and I would be surprised if they *don't* take away the green card if they stay out for more than 6 months. The last officer surely would have put a comment into the system to alert the next officer about their extended traveling. If their next trip is more than 6 months, that puts them into the category of "seeking admission" which invites more scrutiny and makes it easier for them to take away the green card.
If the house is what is holding them up, they need to move to the US and get an agent to sell it for them. And drop the price so it can sell faster. The judge isn't going accept selling the house an excuse when it has taken over 2.5 years. Everybody else sells theirs in less than that or they rent it out.
Maybe it would be better for them to officially surrender their GCs now, and apply for tourist visas. If their green card is involuntarily taken away, it will be very difficult to get a visa to visit, but it's easy to get a tourist visa after surrendering the GC via the I-407 process. They can surrender it and apply for the visa in the same visit to the consulate. Then in the future if/when they're actually ready to live in the US, you or your siblings (if any) can sponsor them again.