"My sister-in-law wears skirts."
Translation:Min svigerinne går i skjørt.
Why is "går i" used here? I have only used "har på seg" until now. Are they interchangeable?
Går i states this is probably a regular occurance, as well as something you can point out right now. Har på seg is the here and now, one instance, "I can see her, she's wearing a skirt."
The plural form is skjørt, like many other monosyllable neuter nouns. However this one has several legal forms, so skjørter would actually be a recognised spelling. Skjørt is by far the most common, though.
Is svigerinna mi also acceptable here? Is there a reason to use one or the other?
Certain dialects prefer one over the other. Having the possessive first was much more common in the Oslo region 30-40 years ago, but has seen a decline. In everyday speak I prefer "svigerinna mi" unless I want to emphasize the possessive, then I'd say "min svigerinne" (I use the masculine possessive for the indefinite form, even though I consider the noun a feminine, it is not considered a grammatical error.)