I feel you are being rather harsh with the OP.
My BF and a friends sis where both diagnosed July last year with breast cancer although my BF also has it in some bones. They both started chemo treatment about the same time and had absolutely horrible side effects. It was for just over 6 months and was once a week for 3 weeks and then a weeks break.
Friends sis had uncontrobable diarrhoea her energy levels were absolutely zero spent most of the time lying down terrible mouth ulcers so found eating if she ever felt like it so difficult and I could go on and on.
BF had the diarrhoea as well plus loads of other symptons. She barely went out of the house for the 6 months except to her chemo and the blood tests. She got fed up with people telling her that various friends of theirs had had chemo and still carried on their lives as normal and that there are pills today that stop all the side effects.
She spoke to her consultant at length about this and he explained that everyones body is different and they react to things differently and yes the pills don't work for everyone. It also depends what strength of chemo and how frequent and long you have it for.
She was also told that she must not mix with her 3 step grandchildren who ranged in ages from 4 -16 as they may be carrying infections. She had to make do with speaking to them on the telephone and on Skype. The consultant said if she had had a young DC of her own would have been different for some reason or other which I cannot remembr.
The OP doesnt know how she is going to be usually takes one or two doses for the side effects to really take affect. I would hate to have other children in the house and suffering with diarrhoea not getting to the toilet in time or panicking if it is not free. Is alright if you have an en suite or a couple in the house.
I presume the OP does a lot of arranging i.e. beds meals etc. when the step DC's are staying with her and her DH so would her DH be willing to do all this if she is laid low? It is all very well saying they could help out but I feel wouldn't be long before they get fed up with that. So in my opinion I don't think YANBU.