Returning resident(SB1) visa chances of success

[ramakrishnachari]

Hello everyone,kindly go through my case
My parents had obtained their greencard in 2009 (under the category of brother and sister of US citizen) ,they visited USA and stayed there for two months and obtained a reentry permit so that my dad can come back to India and take a voluntary retirement from his government job and that procedure would take around a year.they returned back to USA in 2012 two months before the expiry of reentry permit.following that my dad joined over there in a job (in USA) and he worked there for about 6-7 months, but he developed a serious medical illness(chronic back pain and inability to stand ,sit and walk), he visited a doctor in USA initially and found that it would cost more than he expected and more over he was not having a health insurance,so he thought of coming back to India and get it treated(he came with an intention that the treatment may not last for more than six months and he didn't apply for a reentry permit this time),accompanying my dad, even my mother came back to India to take care of him.But later on, events in India led to a prolonged treatment due to the severity of the disorder(spinal disc herniation with lumbar spondylosis complicated by diabetes) he was diagnosed with,the treatment lasted from Dec'2012 to March'2016 which included physiotherapy sessions and bed rest most of the times and a hospital admission for about 10 days( we have saved all the medical reports including MRI and CT reports that are taken during these 3 years and also physiotherapy session cards) and more over he can't sit for more than 30 minutes and the prolonged flight journey that lasts about 18 hours from India was impossible.as we had a strong intention to go back to USA we have saved all medical reports.during this period he used to work from home (just some paper work and general auditing) just to support living.
we are thinking to file a DS117 in American consulate,Mumbai.

will there be any problem to my mom??who had come to India accompanying my dad to take care of??
My sister(19 yrs) is an US citizen(naturalization) and my brother(26yrs) is a GC holder currently staying in chicago
Taking all the things mentioned above please tell me the chances of success for my case