"I love my father-in-law."
Translation:Jeg elsker svigerfar.
It's common to then call one mother "mamma" and the other "mor", and one father "pappa" and the other "far", so it doesn't really change anything as far as possessives go.
If you wanted to use the plural, referring to both at once, I think I would be less inclined to leave out the possessive, but it still works.
In the "working with Norwegians" seminar I was told that "å elske" is only used with very close people (such as family) and the "romantic love" interpretation is really the most common. Clearly it does not map to the use of "love" in English. In Norwegian "å like" would be much preferred, whatever the Duolingo-course says. Same thing with starting a letter or so with "dear" != "kjære"
There are many discusses of that, controversial. Ä like is not strong express. Are there differences between the geographical parts of country or people groups? Does it sound naive or exaggering to over use "elsker"?'"Elsker man iskrem", like in many languages? Whilst, "dear customer" doesn't sound good i.g. in my language, 'Dear Diary'= the journal is very close and important. Men "ä liker" is a weak an unenthusiastic express. We learn the hole time more about Norwegian bokmål! Great!