yep, unconditional love does that! They know that you'll love them no matter what, whereas, aunt and uncle may not come back if they play up.
Happens to us too. We went away for the weekend and I got back to the boys asking for a chocolate freddo frog for going wee/poo on the toilet! Yep, thanks nan for that one. DS1 has been tt for 18 months now and DS2 just needs an incentive for number 2's. So guess who was the bad parent for breaking it to the boys that they weren't getting chocolate for going wees? Yep, me ;)
DS2 will also throw a wobbly when I get him up from his day sleep, but if G'ma or G'pa get him up - totally ok with him.

