If you are doing 10 miles round trip 4 times a day, that is surely adding up to over an hour driving also?
Lots of good advice already, so just a few thoughts of ideas to throw out to DH and SIL.
Could SIL visit twice a week but stay over those 2 nights, so does night and following morning drops?
Could another "cleaner" be found to help out while carer takes her hols? (And she must be allowed take them!).
Is there a community or district nurse who could do them, even once a day or twice a day on alternate days?
Is there any help available from the PIL's GP or surgery? Do they have any nurse available who could do a home visit, even once a day?
Are there any others living near PIL who are nurses or carers or similar, that DH or SIL might trust, who might be willing to do the drops for 1 part of the day, say morning or night before or after their own work, or around their own family commitments? Even just for the short period that 4xdaily is needed.
A longer term solution is needed when 2xdaily is needed though as well. I presume that it's still drops then too? If that is morning and night, the option of SIL staying over on certain agreed nights may be viable to continue.
It does sound though like they need to consider more structured support. Whether that is accepting a carer coming more frequently to the house (we had carers coming to my DGran 3xdaily for 30 mins each - would remind her to take meds, heat her microwave meals (we'd had to run off gas to cooker) and make a cup of tea before they left, with morning visit including personal care) or having to consider moving into a home. Carers don't need to be staying all day with them - my other DGran's carer would come for 4-6 hours once a week, and eventually getting more frequent, to relieve my aunt still living at home - but aunt could do personal care and meds. DGran living alone had different needs so it was frequent daily visits by carer but shorter visits. Big family in the case of DGran with aunt at home so lots of support, by other DGran alone had not much immediate support (Aunt lived an hour away, as did DH and I, DMum lived 3+ hours away, and Uncle lived an hour away and useless - but Aunt, DH, Dmum and I all did what we could to support her) so she got herself into a bit of a mess and had to go into care in hospital and then long stay bed in the home attached.
It certainly sounds like you need to call a family crisis meeting and talk it out in clear terms with your DH and SIL, tell them exactly what it is they are expecting you to do, and how it is unreasonable, and ask what their plans are to look to the current crisis and ongoing longer term needs.
Both the DCs of PIL need to be in the room for that talk and both need to understand the realities and agree the next steps.