"My sister-in-law is kind."
Translation:Svigerinna mi er snill.
'min' is a possessive used for masculine nouns, 'mi' is a possessive used for feminine nouns.
'svigerinnen' is masculine, 'svigerinna' is feminine.
While this is your first mistake, the second one is what errored you: When you put the possessive in front of the noune, you shouldn't decline the noun. Only when it's after the noun should you do this. So "mi svigerinna" and "min svigerinnen" would still be wrong, even if you got both genders correct. Similarly "svigerinne mi" and "svigerinne min" would be wrong.
Huh. I typed in "svigersøsteren min" without thinking, and it was accepted! So do we have the choice of saying either svigerinne or svigersøster? And is one more common than the other? (Frankly, svigersøster is easier to remember since it follows the pattern). Similarly, could we choose to say svigerbror?