Absolutely.
SIL should take responsibility and control HER kids.
The MIL is definitely right to not want kids on her new carpet, especially with colouring pens etc.
I may be considered old fashioned, but you're the parent, and kids should be the responsibility of the parent. The adult should also set boundaries, so when a rule is set, it's followed. I don't believe in... I said so. I believe it's ok to tell a child why, but also that the child should then do as it's told, and face the consequences, should they disobey.
I believe the problems we have with unruly children, is due to parents not following through on punishment, giving in after saying no. If you say... Behave or no McDonalds, then follow it through.
It drives me nuts on the bus when you hear... Sit down or else, sit down or else ... Repeat whole bus journey... WTF no wonder your kid doesn't listen to you, you made this problem yourself.... No wonder when a teacher then says, behave or... The kid just carries on, cos it hasn't been taught consequences. It's the main reason I do t watch soaps, cos they teach, no matter how bad or wrong you are, there's no actions/consequences to their behaviour.