Very generally speaking, if you say something like "We're going to spend the weekend with my in-laws" it means your spouse's family. Probably his/her parents, perhaps his/her sisters.
I, too, wouldn't call my sister's husband "my in-law" without, as saying above, making it "my brother-in-law."
And yes, your sister's husband is your brother-in-law, AND your wife's/husband's brother is ALSO your brother-in-law.