You don't say how long your brother-in-law has been on dialysis, but as a long-time social worker, I can tell you that it is very common for patients to be depressed when they start dialysis or when they have a health setback. I hope he knows that the social worker at his dialysis clinic is available to talk with him and his wife about adjustment to illness issues. Most people think of social workers when they have financial problems, but social workers in dialysis have masters degrees in social worker and specialization in clinical social work. This means that they have training to provide counseling, which is a key reason why the government requires social workers in dialysis clinics. Sometimes paperwork and other responsibilities take them away from this role, but please let him know that he should tell with the social worker he's having a hard time coping. The social worker may know of a support group or it may spur him/her into starting one.
The American Association of Kidney Patients may have a chapter in your brother-in-law's area. Its website is www.aakp.org. Another organization that is patient focused and run by patients is the Renal Support Network (www.renalnetwork.org). On the RSN website under support groups, it has someone in FL he can email.
Another great online support group is Dialysis_support, a listserv of almost 900 members that you can read about or join at http://health.groups.yahoo.com/group/dialysis_support/.
Medical Education Institute that administers this site administers two other sites -- Kidney School and Life Options. Kidney School (www.kidneyschool.org) offers interactive education intended to help people with kidney disease learn how to self-manage and to become partners in their care. There is a module on coping that your brother-in-law could work through at his convenience if he has a computer or you could download the PDF for him to read. The Life Options website (www.lifeoptions.org) offers several booklets and fact sheets to help people and their families cope with kidney disease. One booklet is called New Life, New Hope and there's a fact sheet on depression listed under the free materials that can be downloaded from that site. There are also patient profiles. You might want to suggest that your brother-in-law check out the patient stories on Home Dialysis Central (www.homedialysis.org). There are some pretty inspirational stories there and I'm sure that every one of them had some time that they felt down.
Your sister and brother-in-law are lucky to have you!