Is it permissible to secretly take money from an employer if he delays paying the due wage, claiming kinship and financial difficulties, despite his affluence? And is it obligatory to inform him about that money if he pays the dues?

What the relative did is against Islamic law (Sharia), and it is permissible for the questioner to take from his money an amount equivalent to his right. This is known as the issue of "al-Dhafar" (taking one's right by force or seizing it). There is no obligation to inform him of this, so that it does not lead to greater harm. One can continue working to obtain the full entitlements.