Family tree attached. Names changed obviously!
I've never been very close to DB as he was always pretty horrible to me growing up (constantly calling me fat etc) and I've never really connected with DSIL. But my DD and DN we're both born within a month of each other and I'm keen to try and make the best of whatever relationship we can have. I suspect they both might be only children so it would be nice to have each other.
DB and DSIL sometimes went on family holidays before they were married together with DM, DF, SILM and SILF. DM and DF didn't really like this as they don't like SILM, but they did it a couple of times for DB and DSIL's sake. At DB and DSIL's wedding, DM had an almighty fallout with SILM. DM was trying to hide it so I don't think DB, DSIL and even SILM realise how upset DM is. I don't think DM wants to ever see SILM again (and if they do I reckon she'll need a good decade or two to have passed).
DB and DSIL have suggested a holiday next summer with them, DM, DF, SILM, SILF, me and DH as well as DD and DN of course. DM quickly came up with an excuse on behalf of her and DF why they can't to the place suggested and hasn't revealed that she doesn't want to go anywhere because she hates SILM. I have said that we can't make it this year either. I'm not particularly keen on SILM and DH said he wouldn't go as he finds DSIL and her whole family really weird and difficult.
So here's the thing: I'd quite like to do a family holiday in a couple of years' time with all of our family (the Smiths). I'd like to include our sisters, who weren't included in this suggested holiday. The problem is that because there has been this precedent established of SILM and SILF coming on joint family holidays with us Smiths, I think DB and DSIL (and probably SILM?) will have an expectation that they should be invited too. If we book a hotel with spare rooms I'm sure they will end up gatecrashing, and if we suggest booking a cottage or something they will suggest that we find one with enough space for SILM and SILF.
The obvious solution seems to me to have a very quiet word with DB that DM doesn't like SILM and won't go on anymore holidays with her, but would it be reasonable to do this given that DM is hiding it? It would save DM from having to come up with excuses in the future and I know that she loves the idea of a "Smith" family holiday without the "Jones's" apart from DSIL. I think DB would keep it quiet although may have to tell DSIL I guess and that could drive a wedge between DM and DSIL (not that they are particularly close).