Marriage trouble while dealing with a sick parent

My mom has been suffering from terminal brain cancer for the past five years. Within the past six months it has gotten progressively worse now to the point where she is basically unresponsive and on permanent bed rest. Probably only has a few weeks left. The hardness part about this ordeal is strangely the logistics. I live about 250 miles from my parents so it's not like it's easy to manage helping my dad care for her while juggling everything else. My work has been gracious in that they have set me up with a remote access that I can use to get my work done remotely out of the office. It's been hard to do, but I've managed. It's been hard on my wife too; as she cannot work remotely being a teacher. She has been able to come up with me on weekend trips or times when she has had days off, but 50% of the time I'm apart from her. Lately I think we've had some issues with the separation as she doesn't seem to fair well when I'm not home. We got in a big argument a week ago concerning the furniture (of all things) in our house. I think she escapes grief and problems by buying things, and since her friend bought a new house and newly furnished it, this has become an obsession with her. I basically said that we can look at it, but not right now; I don't have it in me to look for new furniture. End of argument I thought. We never really argue, only about typically stupid stuff,so I'm not sure how to deal with this. Today I left for my parents house again and we got into another argument. She said "I'm never there anymore", and although I can't really help it, it really effects her. I basically told her she was being selfish and unreasonable. I have given her plenty of opportunities to come with me on the trips, to the point on only going up on the weekends to see them, but she keeps saying no, she doesn't want to see my mom like that, or wants me to stay longer than the weekend. It's not like I want to leave; I'd rather be home with her and our normal life, but I feel I have an obligation to help my dad and be there for my ailing mom. Am I the one being unreasonable? I feel like I really need her there for me, but she only cares about me being there for her.