Im not sure why people are having such a go at OP, some of these dynamics are extremely grey and not black and white as others are presenting
Its clear OP and her husband would have wanted different care for the mum but that cant be forced on any one and the brother didnt want that to happen so her husband couldnt force that
She's not in control of who her parents invite or dont invite to dinner and coming out with 'I wouldnt have a problem with x random strangers coming to dinner' well thats not the point, her parents do, she cant just rock up with someone they dont feel comfortable with for some reason
What you could have done OP is start the conversation a lot earlier in the year becuase theres a few things missing from your post
Does the Brother in law want to be with family or want to be on his own, does he have other friends or associates he can spend time with, is he planning to go to the pub or something like that. Near us there is a small ethnic cafe that is open today, thats where we are going
Is the brother in law likely to be offensive at a familly gathering, does he pose risk to anyone, is he likely to be foul or anti social? If none of those perhaps prepare your parents earlier in the year, but they are not obliged to have people there they dont want
Your last option is that you hold xmas at yours, for a much smaller num ber including him and boxing day you go to your parents. Sounds as if that would actually be better for your parents in any case because they probably get stressed xmas day