Why don't you and your SIL come to an arrangement where you both take time off work and watch each other's kids? And leave your MIL to enjoy the peace! Maybe you take 1 day off a week and watch all 6, then your hubby, then your SIL, then BIL, then on the remaining day. On the 5th day maybe you're mother in law can mind the 4.
There is no way in hell I would mind 6 kids. NO WAY IN HELL!!! Let alone expecting an older woman to do it! I would never leave my kids alone when my mum was minding 6, no way she want watch them all. It would be completely exhausting.