When does it become acceptable to draw the line the with caregiving when family won't help?

And the stress starts to cause depression. My husband and I are the caregiver to my mother in law in early stages of Dementia. In a matter of 1.5 years we have each lost a parent and have had to make living, medical and financial arrangements for the remaining two who are unable to do these things. We have stressful jobs and other issues that require our attention. The days off of work, canceled plans and life on hold have added up.

My husband's only sibling lives out of the country w/ a spouse and two kids. We are local and have no kids. My husband's sibling refuses to help make calls or emails regarding her care - things that can be done remotely. My mother in law is in a very high maintenance stage - still able to function and makes many requests/complaints about her assisted living situation even though she is the very best place in a major city. The calls are constant. She knows we are stressed and her questions are becoming a bit intrusive about our personal lives.

We have taken on everything because we are close and we want the best for her. The opinion of the family also seems to be that we are local and have no kids, so this should all be on us. Every complaint from my mother in law comes to us because she doesn't want to bother the other sibling who has a family and lives far away.

Having exhausted every avenue asking for help from his family and putting her in the most expensive facility.The situation has gotten to the point where we are headed for divorce and a nervous breakdown. I am out of options. To make matters worse, there is the expectation with his sibling that despite the unwillingness to help us, we should have a fun relationship where we all shoot the breeze on holidays via phone.

When is it acceptable to draw the line with mother in law and the sibling? I want my husband to have a good relationship with his family, but not at the expense of his mental and physical health. How do we reclaim our life? We know this is just the beginning of a long road ahead with far more complicate issues as the Dementia advances. Advice?