Your husband will not say no. That is your problem. Not the in-laws.
Is there something relevant from a cultural standpoint that is influencing these behaviors? For example, is he the oldest son and doing well in a family that traditionally expects one person to support everyone? Does he brag about his big job and big salary, so everyone thinks he has more money than he does? Does saying "yes" make him feel like a big success, like someone important? Why is this so essential to his self-worth?
These are not loans, right? He is giving the money with no reasonable expectation that he will be paid back? No contract, no written agreement, no partnership in the businesses and no say in how they are run? No business plans, etc.?
Do you work? Do you have income? Can you put that money aside for your baby's education and possible medical needs, and live off your husband's salary? That's a start.
But essentially, this is a huge problem between you and your husband. I'd ask him who you two would borrow from if you needed it - who in the family is there to support you? If he thinks nothing bad can ever happen to him, ask him why bad things have happened to every single person in his family, from brothers to cousins.
Go to marriage counseling. If he won't go, go by yourself, and figure out how to get a financial advisor to help you plan for contingencies. This may be your best hope.