Far too many variables in this situation for any of us on MN to make comment
How much did MIL give to SIL to help buy house as in it is that she sold her home to give to SIL to buy and have part ownership of SIL home? What has happened to MIL home? Did OP DH get any help buying their home in the past?
SIL and BIL finances could have changed since buying the house and need to Air BnB to make ends meet.
BIL family coming and staying, told known if they pay or help in other ways or stay in lounge.
Not seeing MIL because she lives with SIL who uses her spare rooms for income does not mean you don’t see MIL. With adult DC, can’t they share the driving to bring MIL to your home? (OP said they're all grown)
As adults, can’t they go on their own to visit too? There’s options of staying nearby in b&b or inexpensive hotel?
Another option is to collect her and stay together someplace else altogether, so lots of possibilities to see MIL that don’t involve SIL at all.
A new “normal” of seeing MIL needs to be found now that MIL is no longer living in her own home with spare room. There must have been a reason why MIL moved in with SIL prior to covid (if covid was not at a factor in MIL moving).