April has 30 days, so if he didn't walk the dog on Sundays, he worked for a total of 30 days - 4 days = <<30-4=26>>26 days.
If each day he earned $10, he ends up with 26 days * $10/day = $<<26*10=260>>260 .
After buying books and giving his sister $50, he ends up with $260 - $50 - $50 = $<<260-50-50=160>>160
#### 160