Is there sin on the granddaughter who changes her grandmother’s clothes without her knowledge and consent, given that the grandmother does not object to that?

What appears is that there is no harm in your mother's action of exchanging these clothes for clean ones, as there is a benefit for the grandmother in this, especially since she does not object to this exchange. This may indicate that she has been afflicted with some dementia, due to which she is not fully aware of some of her actions.