My ....... (mother-in-law) cooking. Put mother-in-law in a genetic case

Menna Basha
I believe by "genetic" you mean "genitive," which is the English case for showing possession (indeed, this is also called the "the possessive case" or just "the possessive").
The possessive of "mother-in-law" is " mother-in-law's ," with the " 's ," which is the usual case marker of the genitive in English, appended to "law" and not "mother" since even though the referent of the compound noun "mother-in-law" is a human female who is the mother of one's spouse and not the abstract noun "law," the components of this compound happen to be arranged such that "law" is the last or rightmost element of the stem and thus is the recipient of any inflectional suffixes (cf. plural "mother-in-laws" and plural genitive " mother-in-laws' ," alongside "mother's" and " mothers' ").

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