Interesting point - I would agree that if one family was five and you had to get them a "family" size room on the ship and the other family was four so could fit in a regular size room, that would fair because there's a difference in the family's needs. But here, since the SIL has kids, it is not like she needs less space, or would use the verandah any less. Really I guess it is more like the difference between Savannah View and Standard View, or business class versus coach on an airplane--you are paying more for a nicer/upgraded in-room experience, without any change based on the family's individual needs, so I still think the more polite thing to do is to get the same for your guests.
I agree in this case with your analogy of Savannah view vs Standard View since space isn't the issue here.
I guess my point with the studio vs 1 bedroom is that, it accommodates the same number of guests, even though the 1 bedroom is more spacious. I don't feel I would be treating them poorly if the family members were in a studio while I'm in 1 bedroom...especially if I've always booked a 1 bedroom.
It really does depends on the attitude within the family members. I wouldn't feel bad about inviting them to stay in a studio because they are perfectly happy and quite excited to do so. They wouldn't accept an upgrade even if I insisted.
Even if I invited someone that wasn't a family member (although I wouldn't do that), my main concern would be to ensure that they get an appropriate stateroom for their family size, but I still won't feel the need to get the same category room for everyone. If we were invited, and we wanted a better room, we would insist on paying for an upgrade. Although this would be a bigger issue for me if they were not relatives. I would feel uncomfortable upgrading if the person paying was not. On the other hand, my relatives know that we may upgrade and they are perfectly fine with it.