I wanted to treat my mum for her birthday so me and dh took her out to theatre and for dinner, our treat and a present. As we couldn't get the tickets for actual day of birthday and mum wanted to be busy she asked if I would go to west end with her and sister and have lunch. Just got message from sis asking if I wanted to split cost of lunch at expensive restaurant, aibu to say no I have already spent a lot? Tell me straight, I know it will lead to argument with sister!