How do you get thru to sibling you need a break every other month? Feeling taken advantage of!

My 86 year old father in law who suffers from Alzheimer's has been living with us for the past two years. The first 8 months he was with us we would take him daily to visit his wife in the nursing home until she passed. The first month we drove over an hour each way and visited for a couple hours after working long days full time. After several falls, we moved my step mother to a more local nursing facility and they provided excellent care. It helped tremendously to only travel 15 minutes one way. My husband would drop him off after he go home from work and I would pick him up and visit with her around 9pm daily. They had been together for 50 years and we knew how important it was for them to be together as much as possible. Before, she needed more extensive care, they were living in their house together which was also about an hour away from us. She needed extra care so we hired in home 24/7 care for both of the for about 3 months. That cost them close to $30,000. But, even with that care we needed to call daily and go over a few times a week. And you had to check on these hired caregivers. Also, both of us were working full time.
Over the past 2 years time my brother in law has taken my father in law maybe a total of 3 months to his house. If it was the summer months they go to the campground where my father in law has an RV (my brother in law talked him into putting down there near his camper) Everytime, we have had to ask when he could take his father for a visit to give my husband and I a break. Never has he offered on his own. My husband retired a year ago and has not had much of a retirement. Since my step mother in law passed, we slowly see my father in law's dementia getting worse but he is still mobile and is not a problem. We enjoy his company and know we are doing what is best for him at this time. However, a caregiver needs a break. I am still working full time and my husband is in his mid 60's and his health is suffering. Not only do we do the daily 3 plus meals, medications, bathing (thank God he can shower himself still but you have to make him do it), paying bills, cleaning out their house to get ready to sell, taking care of all needed financial and medical needs, etc. Plus, we do not take a dime from him for rent or care giving. It has only been in the last month we have him paying toward groceries (mostly because our food bill has gone up because he wants stuff we didnt usually buy. My husband is at his breaking point with his brother because he says he will take his Dad and then some job comes up or some other excuse and it wont be for another 2 months. My brother in law say well you had him come live with you and just throws his hands up in the air. He said there is a facility down by him we could put his Dad in. The problem is that we are not at that point yet. It may come some day but not yet. We know very well we had him come live with us but all we are asking for is some help. Every other month take him for a visit for a week. My brother in law tells my husband well you're retired. I just want to say no he isn't- a full time care giver is a job!! And not one he was looking forward when he retired. We have not taken a vacation in over 2 years. We never have time alone with each other. We have not attended many events (weddings, parties etc) because of his father. We just need a break! So how do we get through to his brother that their father is also his responsibility. We need him to pick up some slack. He takes vacations, uses their fathers RV, golfs, relaxes and takes time for his wife's side of the family. His job entails 2 days of travel and 2 days of work a week maybe 3 weeks out of the month. Asking for advice from others who may have experienced similiar situations.