I am constantly inundated with requests to babysit, from my mum, my MIL and my aunt, my sister, my friends etc. At first I had a great excuse that C was exclusively BFed, but now I just say "no thanks, I'm here to look after him and I enjoy it". They still ask now, and maybe one day I'll take them up on the offer, but until then, I just keep giving them that same response.