How do I deal with the cat issue without it becoming an argument every year?

My parents, who live with us, insist we come home from our cottage, 7 hours away, to cat sit while they go on vacation. My husband and I have recently retired and started spending more time at our cottage, 7 hours from home. We are farmers and we move north as soon as the crops are planted and we return for the fall harvest which is determined by the weather. My parents live with us and they also want to take several weeks away in the summer which is perfectly reasonable. The problem is they have a cat and they insist we come home to look after the kitty when they go away. I've asked them to have someone come in or my son has offered to look after the cat at his house but they refuse either option. This year we are cutting our summer short to come home to look after their cat because my parents are going abroad for 3 weeks, same last year. I know it sounds selfish but I have health issues and maximizing my time at the place I love is important to me. My parents think we're being selfish because they "look after" the house while we're gone and they don't think it's asking too much for us to come home and cat sit. In reality, we would have no problem if they left for the whole summer. We've made arrangements for someone to cut the grass while we're gone so all they need to do is take out their own garbage and they are in no way obligated to take care of the house. We also have a cat and she lives happily with my daughter while we're away and our dogs come with us. H