I have DD16 and DS9. DS9 sees his Dad 5 days a month. I see friends / plan things for those days. Occasionally something falls on an evening when I have DS so I will ask DD to look after him, although this doesn't happen often.
I don't have a good relationship with DS' Dad and communication is kept to a minimum.
This week it was my friends birthday and she had planned a meal. It was a week night so I had DS. I asked DD to sit. I left at 7, she put him to bed at the usual time and then I was home at 11. DS was asleep.
DS has just returned from his Dad's and has told me that his Dad is furious that I left DS in DD's care and he will be having serious words next time he sees me. DS had told his Dad that his sister had told him off once for having to be asked to brush his teeth repeatedly.
I've asked family and they all say that it was fine to leave DD babysitting but would be interested to hear what others think as DS's Dad seems to be incredibly angry about it!