Kind of wonder if step dad would give you or someone else a jump start every couple of days?
It doesn’t hurt your battery, but if you make a mistake sometime and cross the wires up it could knock out your alternator or PCM. Is step dad going to pay for those kinds of repairs?
If he can’t buy a new battery then you should buy a jump start pack and keep it charged and handy. Use that to fire up jump his car.
If you keep doing this service for him he won’t need to ever buy a new battery. The risk of damage to your vehicle goes up every time you do him this “favor”.