According to the Ministry of Manpower, a maid is not allowed to work for another person or at another address.
https://www.helperchoice.com/c/singapore/job-duties-of-domes...
https://www.helperchoice.com/c/singapore/job-duties-of-domes...
In this case, they thought incorrectly and have been caught, exposed and worse.
However, I doubt that true justice (as in legal punishment of some sort) will ever be meted out to the rich and powerful family.