Just because he didn't like the idea of selling, doesn't mean they have to obey his wishes. They absolutely should discuss it, as being a landlord isn't everyone's cup of tea and they should not be forced into dealing with renting/managing property out of some sense of misguided loyalty to a loved one's memory. It is just a house; it's not a shrine, and they need to do what is best for them.
That being said, you can offer to help them clean it out, they could hire a cleaning/organizing crew to do it for them (depending on where they are located, 800-Got-Junk is a packrat/hoarder group that will come out and remove and haul off stuff if the house is that level of filled up, otherwise they could get an auction house in to sell off the contents and/or ask family members to help them. But again, all you can do is offer your help/other options but they have to take you up on it. (I just assume it may be really junked up but I had to deal with a terrible hoarder house situation when my dad died, and it took around 2 years to get it cleaned out, fixed up and sold).
Their father's death might be hitting them particularly hard, and that's why it's taking them some time to work on the house and why they're not considering selling it right now. If it will take them years longer to even get the house cleaned out, I would likely wait til the 2 year mark and then offer to help again - cleaning out/organizing and remind them that GFIL may have wanted them to keep the property, but not at the cost of their own financials and well-being, so selling the property might be something to take another look at...