Your spouse’s female siblings are not your sister-in-laws, but your sisters-in-law. The same pattern applies to brothers-in-law, fathers-in-law, and mothers-in-law.
The new edition of Common Errors in English Usage is now shipping, and it makes a great gift. When you order 5–9 copies and use the coupon code FIFTEEN, you pay only $13.48 each. Order 10 or more for just $12.15 each. All with free shipping in the US. Single-copy shoppers can still use the coupon code and get the new edition for $15.