Why the guilt seems right
Let me start by conceding what's true, because a book that told you the guilt was baseless would be wrong, and you'd know it.
They raised you. Whatever else is true, somebody fed you and got you to school and sat up when you were ill, and for most readers that somebody is the person who now can't manage the stairs. The debt isn't imaginary. There's a real asymmetry in what was given, and a real pull to give something back, and I'm not going to argue you out of it.
They're frail, and it's getting worse, and there's nobody else. Or there is somebody else, and they're doing less. The need is real. If you didn't go, the milk would be off and the television would stay broken and the loneliness would be worse. Some of the should is simply an accurate description of what would help.
And the time is finite. You know that. Every visit is one of a number, and the number is going down, and you don't know what it is. That thought alone can turn any afternoon you didn't go into something you'll regret. Guilt is, among other things, a way of taking the finiteness seriously.
There's one more thing the guilt does, and it's the one that keeps it in place. It feels like the thing that makes you go. Take it away, the fear says, and you'd drift. You'd ring less. You'd become your brother. The guilt seems to be the engine, and a bad engine is better than none.
So the guilt has four jobs. It honours the debt, it answers the need, it respects the time, and it drives the visits. Those are real jobs. The question this book asks is whether guilt does them well, and the answer, from the research and from watching what guilt-driven care becomes, is that it does the first three badly and the fourth at a cost that's mostly hidden until it isn't.