dilemma

Ladies need advice! All views welcome! My 7 year old visits with her grandmother some weekends (myself and her dad are split since she was one and he never has much to do with her his mother has always took on the father roll and has always been amazing) my daughter was meant to go down for a sleepover this weekend but I put it off as the granny's daughter (my daughters Aunty who's 18 and just moved into her own place) wanted her to stay at her house. I'm not comfortable with this one bit , the girl is lazy and incompetent and only wants my daughter to entertain her child who is one.. To be honest I can't stand the girl.. However I haven't voiced my concerns with her grandmother as I really don't want to offend her in any way because she is so good to me and my daughter, I'm just wondering do any of you ladies have any suggestions as to what I should say without being offensive, I know it will come back up again soon and want to be prepared. I'm such a wimp I hate offending people especially when they have always been so kind