I've been thinking about this and am in a similar position to the OP.
If DP and I visit my DM and his DM does that stop them having indoor contact with other people because they're both in our 3 household bubble?
We all live in the same city that is likely to be in tier 3 unless there has been substantial reduction in the numbers during lockdown which I don't think is the case. No-one is going to be staying overnight, it would probably just be visits for a couple of hours or so to catch up and exchange gifts.
Or can my DM look at it from her point of view and not count MIL as part of her 3 household bubble because she won't meet her? I'm genuinely confused on this point. BBC website says DM and MIL can't meet other people because they're in our bubble but I'm not sure I can see anything on gov.uk that says this isn't allowed.
Anyway, I've done a diagram for illustration.
A = DP and I
B = DM (widow, lives alone) and her support bubble which is dsis and her family
C = dsis adult DS and his family, which DM/DSis are likely to want to meet, I would normally but obviously not this year.
X = then it gets messy because adult DS dad is Dsis ex so that's a different household and then you have to consider his DW side of the family - quite big and normally extended.
On the other side
D = MIL who has two siblings that she would normally see, but probably not this year unless it's an outside walk in the park. MIL is married to her second DH, who doesn't really have any family as far as I know so they are likely to choose us and her other DS, E
E = BIL and SIL, who will want to see MIL but can live without meeting us. However SIL family is also complex, she has lots of siblings, adult DD and family and her parents, so I don't know what they'll do, especially as seeing many of these would require travel and overnight stays.
So Venn diagrams rather than bubbles. I suspect not strictly within the rules but I think that's probably what will happen in our family, for A to E at least. The risk is probably low enough for a couple of short visits if we're careful, both our DMs have barely been out anyway and nowhere inside and crowded.