How old is your DSS? It obviously sounds like you live quite a distance away, and that is also the situation with me and my partner when compared to DSS's mum.
We had similar issues not so long back, and I spoke to her (DH doesn't have the balls, he has deep issues with her due to how they split). There is a court order in place, to say that we have him in the week, and she has him every other weekend.. but she was regularly asking could we take him to hers, or pick her up, when she is supposed to do that, and she chose to move an hour and a half away knowing she would have to pick him up and take him home. I didn't think it was fair for us to have to make a 3 hour trip (not to mention the cost of fuel) at the last minute because of whatever she was doing.
I made it clear to her that when it was OUR time, WE were responsible for taking care of him, and would not ask for her help. But in return, she must expect the same. So, on the weekends he is with her, she is responsible for him, and if she is unable, she must make arrangements with her family or whatever, like we would ask either mine or my partner's mums when he's in our care if we needed help with anything.
That worked quite well. Perhaps you need to set in stone with her the arrangements, so that she is responsible for him at some times, you at others, and she doesn't call on you when it is her time.
As you said, priorities. Or, if she is unable to do something, she has to do what every other parent does and find a way around it, without putting other people out when it isn't their responsibility at that point.