Do you know if your mother-in-law is taking dexamethasone (Decadron)? It's a steroid medication commonly given to GBM patients to control swelling in the brain. This drug can have major side-effects changing the patient's mood and personality. My father has been taking dexamethasone since his diagnosis in January. At higher levels it turned him into a "crazy-person". Angry, agitated, paranoid, etc. It was very hard to live with. His dose is getting lower and lower and this side-effect has gotten much better. The doctors also added risperdal to help manage this.
Anyway, that's just one possible explanation.
My advice is to try not to take it personally. They are going through a lot and won't react in any way you might find reasonable or rational. It's no one's fault. All your efforts to make the visit easier for them might seem unappreciated, but it really has nothing to do with you. Don't feel bad.
Perhaps visit alone, just you, or your husband. Don't stay long. Only bring the children if your mother-in-law asks for that. It's extra work for you, and might be overwhelming for her. And don't be offended if they might not want to visit you. You never know, it might be an exhausting effort for them to get ready to go somewhere. In my dad's case, just getting ready and going to a doctor visit could wipe him out for the whole day.
Just some thoughts. Good luck to you.