Cleaning my brothers house?

i went to my brother and sister in laws house today and it is filthy and messy. It never used to be and i have seen things getting mucky in their house over the past few months and in a kind roundabout way said about keeping it tidy. Anyway have just had a discussion with my husband about me going over and blitzing the place and he said said a full blown absolutely no way. Would you go to your families house and clean or not, it's not costing me anything but time and i am willing to give up my day off just so that they can have a clean house. If they have a clean home surely other things will feel better in their lifes.