I think what Jemisa means is that if you have a daughter, then your son's wife will be your daughter's sister-in-law, e.g. my husband's brother's wife is my sister-in-law.
On the other hand, my sister is no relation at all to my husband's mother.
No she is not. Your sister-in-law can only be your brother’s wife or your husband’s sister. The “relation” you describe is actually no relation at all to you. As your husband’s brother’s wife she is your husband’s sister-in-law but not yours. The relationship is one stage removed from you in the same way that Mampara’s son-in-law’s sister is.