It is really important that you draw the lines where you want them to be. Not wanting to look after your GD is nothing to do with not caring/not loving etc. It is entirely reasonable that you want to build a happy relationship with your granddaughter over the years looking after her twice weekly. Other than that, I assume you would happily do extra in a real emergency. But for the rest it is also entirely reasonable that you want your own free time, other things in your life etc.
I think you need to be clear and say that you love the 2 days regular childcare but it is tiring and can't manage extra unless it is an exceptional emergency. And stick to it.
When Is News Not Really 'News'?
When a political leader lies on their CV - can you trust them?
