It's been a year or so since I researched phone service and so there may be a less expensive way for them to obtain service on their own but I doubt it. I suggest that the best way to help them would be to add them to your plan. My daughter and son-in-law have his mother on their plan and it doesn't cost them any extra, probably because they made use of a "special offer." She received a new phone when they enrolled her. I wanted the extras and so have my own plan.
I suggest that you don't give in to your brother's unhappiness over adding them. I don't see how you could be too involved. They've asked you for help. It's reasonable that you help them. I wonder if he's relying on them paying to help him with his bill. I very much doubt that it costs $80 extra to have them on his plan. I suggest that if he's annoyed it's because you've stopped him from taking advantage of them.
When I was looking for a plan, I could go with a Verizon, basic service for one phone, for $20-30. It would not cost double that for 2 phones on the same plan.
I googled cell phone plans comparison chart and found this site. There were several sites available.
http://www.myrateplan.com/wireless_plans/