Lakelover I'm sorry things have got this bad for you, it must be really hard to deal with. You do know that it isn't because they want to exclude you, it just that horrible though this is they don't 'see' you - you are son's wife, grandsons mother - and not Laklover in her own right - I doubt it's deliberate or that they are even aware they're doing it. I know that doesn't make it any easier to deal with, but hopefully it will confirm that there's nothing you can do to change how you re dealing with it that will make any difference. As in, you can't change them, you can only change how you think about it. You count, you're his mother, they cant take him from you in any way no matter how much he looks like your husband. I think you need to focus more on dealing with your feelings of insecurity about this, rather than focusing it all on them and hoping it will change- as I said, you can't change them. What you can do is face them with dignity, grace and calm - because that's on you. if they are rude and unpleasant, that's on them. It's only a visit, they're not moving in.
If it helps, I have a friend whose MIL was so awful she refused to even speak english ( she was French) in front of her even though she was fluent. When it came to a group family photo one Christmas, the MIL cut out a photo of her ex DILS face and pasted it over my friends face on the picture. I'm not belittling what you're dealing with, but telling you this because it could be much, much worse.
Good luck with it all, x