Never ever ever ever let them know your pay. My husband made the mistake of not keeping his payslip properly when he first started working & they demanded 50% of his pay bcos he was 'single' & did not have to pay any bills! But we were saving up for our wedding so he ask company to bank all his pay into our joint acct instead.
My parents know how much we are earning. Well, she's the one signing my husband's paycheck & my pay can be guessed rather easily by calculating how many years I've worked. But my mum also knows we have alot of bills to pay, so she's never asked for a single cent from us. Even family holidays she'll pay for us & tell us to save the money for my son.
If your ILs are understanding, can let them know roughly which range your pay is in (but always declare lower!).