So you don't feel you should anything for your mum because she looks after your sisters children and not yours?
Honestly OP either do things for her because you have time or want to. Not because you want something back.
I worked ft mum helped with my first but not my second. She helps dbro and his wife far more than she helped me despite sil being a sahm . At least once a day she or my dad are asked to go up and help with something
However sil and dbro are very critical of her and only contact her when he wants something.
She would ask me for a favour before them and I don't mind. She does what she does for dbro and sil, because every time she says no it's a huge drama and she knows they aren't above stopping her seeing the kids.
I, personally, know that would last than a week because they are totally dependent on mum and dad. If one of them has a doctors appointment mim or dad has to have the kids so they can go together. Even though both drive. I am not talking appointments where one of them is really ill and can't drive. Just general 'I have a cold' type things.
Or a few weeks ago they called mum as the oldest was sick. There ended up being 4 adults looking after 2 kids. Because dbro and sils first reaction is to call mum and dad.
But I am the one mum and dad have a better relationship with. The relationship with dbro and sil is very precarious and mum has started standing up to them a bit.
I am happy with it the way it is because my mum and dad respect me for being independent. I enjoy helping them out and enjoy the better relationship.