Depending on the distance of where the relative lives it is never just half an hour out of your day. Depending on what time you can go, I go after work this can be anytime between 4:30pm-7:30pm, there is travel, do you need to pop anywhere first to pick anything up for them. Depending on how they are depends on the length of visit.
Once you get home there is still your own family/pets/home/ to get on with. It all sounds so easy to do when you read it but in fact it is never that easy in reality.
If they are not of good health, it is also emotionally difficult/exhausting.
Op I think you should look at suggestions above, identify Grans needs, speak to DB and DH and go from there. If Gran is still of sound mind she will understand if you talk to her.
The long term daily solutions can wait until your parents get back. We suggested to FIL as he was very resistant to carers that they were coming to do the cleaning and then any other jobs he could think of. He said he could manage and then we said about the ironing, vacuuming the stairs, mopping floors can he manage this. Once he accepted the hour visit in the morning he was a lot more receptive to further visits per day when he needed them.
Ignore the harsh responses - you do obviously care very much about your Gran and sound torn between your own children's needs which have not happened before - eg starting secondary school, not settling into a new year and then the business. If the children had just slotted back into the new school year I do not think this would have been such an issue. The visits and business you would have managed as you know it is for 3 weeks. I think you are trying to keep all the usual plates spinning but this time a couple of extra plates have been added and they are beginning to wobble IYSWIM