we can do when it comes to our siblings not paying enough attention to elderly parents.
Regarding your parents, since they're in good health, is there any hobby they can take up to get them out of the house? Are there any senior centers around or any church groups or anything like that? It doesn't make up for the sting that is adult children (and grandchildren) not visiting but it would help break up the monotony. If there's no established groups your parents could join--birdwatchers or senior hikers or whatever--maybe they could go online and see if there's a FB group that meets in their area?
Regarding both sets of parents, do the other siblings know how your folks, and your husband's folks, feel? Maybe they feel the amount of visiting they do is fine and don't realize the parents would like to see them and any grandkids more often.
I'm sorry, hun...what a frustrating situation all around. I hope you can find something to keep all the seniors more engaged and I hope your siblings and your husband's siblings step up and relieve you of some of the effort of being your parents' main source of socializing.