Nansue You just have to ignore it and if she has asked for poo for lunch just say ok and carry on. Give her whatever you think and when she questions it say that is what it is - and lets face it at the end of the day it is! Just accept whatever she says and do what ever you think appropriate if she is still giving silly answers. Don't get cross or show any interest whatsoever as you want her to get the idea she is boring and you aren't shocked. It could be just be something she heard which tickled her funny bone and she loves the reactions she gets by using it.
It is of course possible that it was big brother and also possible that there is a fair bit of jealousy going on there in which case he probably needs big hugs from Nansue as it is tough learning to share everything with younger siblings.
My DMIL told me that DH's older brother did the same to DH and the first time DH came out with the F word he was on the bus with his mum and asked in a very loud voice, "What does F mean Mummy?" It was in the late 1940s and as you can imagine she was mortified. Big brother had learnt it at school and knew it was bad and was trying to get little brother into trouble. It never changed and even to this day DH's brother, now nearly eighty, is jealous and displays all that goes with that. 