That's a crap situation to be in.
First off, I think when you discuss this with your husband you need to remove all emotion from it (easier said than done I know). He is probably going to be very emotional about it (using words like "hate", "unfair", etc). You need to be neutral and factual.
"No, I don't hate your mother, I just don't want to live with her for a whole year." Don't discuss why you don't want to because he's always going to bring it down to "you hate his mother".
Try and think of arguments that are reasonable and that he can't object to. For instance, how big is the in-law's house ? It'd have to be quite big for it to be reasonable to bring in five extra people for a whole year.
And then focus on the practical side of it. Can you afford to keep going as you are ? If not, can you save money somewhere else to make it possible ? If it's not at all affordable to keep going you may not have much of a choice, but otherwise you're not unreasonable to say no.
Your husband may be the sole earner but you are, I'm guessing, doing the bulk of childcare and home admin/chores. That has value. Your husband doesn't get to call the shots because he is the one earning. If you were the one earning, would you tell him that it's your money and he's being ungrateful ? Or would you consider that you're a team ?