Where do you put the apostrophe in sister-in-law?

There is NO apostrophe in 'sister-in-law'.

However, if you have two brothers who are both married, then you will have two 'sisters-in-law'. Note the position of the plural 's' and no apostrophe.

However, if you refer to their coats, then it becomes 'sister's-in-law coats'.

Again note the position of the 's' and the apostrophe; now used possessively.