I like the direct approach. Since you're not that concerned with keeping good relations with the in-laws, it only makes it easier to say this with the right tone. The next time they make a thinly-veiled insult:
Your comments about my housekeeping are getting really tiresome...
If they start to look like they are not taking your seriously:
...no, really. I'm tried to be nice. I've tried to give you the benefit of a doubt that you're not making underhanded insults. But I'm at the point where I really can't take it any more.
Now put them on the spot for an answer:
Can I rely on your to stop making these sort of comments about my housekeeping?
At this point they are likely to back-peddle, and comment that they were only joking, etc. That's fine. But don't let them avoid the question:
That's all well and good. Thank you for telling me. Can I rely on you to stop insulting my housekeeping?
Repeat the last sentence as often as needed until they provide an answer that indicates they have taken you seriously.