My sister and her DS came to stay last week for a couple of days which I had been dreading because her DS is a bit of a handful and I'm 33wks pregnant. I have a DS (5) and DD (3 last week) and my DH works long days with an hour plus commute.
The whole time my sister and her DS were here he got away with murder because she lets him. Her DS (six weeks younger than my DD) is a stocky boy and would push and hit my DC and sis did nothing. In the end I did all the telling off. But then she would coddle him and say "it's just that you're heavy handed, not everyone plays like you" or her DS would snatch and whine and be generally unpleasant and she'd look at me feebly and say "he doesn't understand" or "he's just tired". My DD is just 6 weeks older and understands perfectly when she's told not to do something or not to bash a child or they will be consequences.
He has done things like punched my DS, picked up a toy and hit him across the head with it, pushed my DD off a step ladder then lied about it (I know she shouldn't have been on it) 
, broken or taken toys that were put away (even though there's a garden and living room full of toys he can play with), and general snatching, shouting, not listening and answering back to me to name a few.
AIBU to not want them to visit again until attitudes towards bad behaviour are addressed?