How do I get some time to myself away from my family?

Sounds bad I know, but I have been helping look after my brother's children, 8yrs, 2yrs and 7mths for 2 weeks and I need some time out. Their mother gets back today and now my mother wants to spend the day doing things but I want to go home and chill in my spa. What do I say to get away? I realize this might sound selfish but 2 weeks of cleaning, cooking, changing nappies, averting disasters and feedings has worn me out. I'm not cut out to be a mother I don't know how they do it but the last thing I want to do today is everything my mother wants to do. How do I tell her without hurting her feelings?