Niece

My mum looks after her grandaughter on Monday and Tuesday afternoon from one each day (sometimes we pick her up early). My sister puts her in nursery in the morning and we pick her up from there so my sister can go to work. When I am off work I help my mum with her. Today we pick her up as usual and we go out with her give her some food with us in a wetherspoons and we look round shops and put her to bed. we go to the dentist and my sister rings us asking why she is in different pants. i changed her nappy but pants were ok so never changed them so the nursery must have and asked why she had a mark on her cheek i didnt notice a mark anywhere while she was with us then asks where her shoes are. she is one we never put shoes on her at all so we never noticed they were missing. we have been doing this for a few months and she barely even thanks us. my mum has now taken the hump because she feels my sister doesnt trust her. my mum has taken semi retirement to help my sister she doesnt offer her any money which is fine and will watch her at the drop of a hat for my sister. and dont know what to do about it now