I'm the oldest and I was never paid. It never crossed my mind that I should be paid because I was raised properly - that's my opinion on this

It's just two siblings staying home alone together...the older isn't actually babysitting as in entertaining the younger one...they are just home together. This is something older siblings do occasionally and it sends the absolute wrong message to pay for this. Never ever would I pay my older to watch the younger. They are most likely both excited to be home alone and have the house to themselves. That's the pay. No money needed.
If you live somewhere where they can walk to a grocery store together you could leave them 5 bucks to go and buy themselves some treats for their afternoon home alone. Or go with them the day before and buy them some chips to share. Something like that I'd consider but certainly not paying anyone.