Hi T.---I have a couple of thoughts for you to consider. We have always lived far away from my kids grandparents. So they've never had the opportunity to spend much time alone with them, by themselves or all together. Cherish that your kids get to see grandparents on a regular basis.
Your kids are small. They require more effort for their care. I would think that it would be more than fair and acceptable that they visit individually, provided the other child gets an opportunity at another time and place.
Having said that, it is important for your boys to learn as soon as possible that all in life is not fair. Not that one should be excluded over another, but that there are certain priveledges that come with being the oldest child, and right now that seems to be extra time with grandpa and grandma up north. The baby doesn't know about all of this so it doesn't matter to him right now. Don't project your fears of unequal time with their grandparents on the kids. You may set them up for a rivalry when there is none. It's just a matter of timing right now. Things are likely to change as they get older. As long as grandma and grandpa also spend time with your youngest at other times and locations, I would think that is appropriate considering the ages of your kids.
When my kids were younger, my in-laws would take each of them, one at a time, one summer at a time, to spend a week with them in Iowa at 'horse camp'. They have horses and wanted to share that experience with them. But they only would have one grandchild at a time for safety reasons. We have grandchildren #'s 1, 2, and 5. So our youngest had to wait 5 years for his turn. That's just the way it was.
Things are not always equal, but we can work to make them fair for all. It takes cooperation and compromise. I pray that you can repair the rift between you and your husband's parents. Remember, it's not about you and it's not just about equal time with grandpa and grandma.
So, unless your in-laws are showing preference for your older child at times other than trips up north, I wouldn't worry about it. Good luck. D.