should't we say jeg liker ikke svigermor min to be correct its not specifing whose mother in law is that
When the possessive is omitted, it's assumed that the subject is referring to their own relative.
PS: If you want to include the possessive, you need to say either "svigermoren min" or "min svigermor.
In negations like above, it means "either":
"Jeg liker ikke X."
"I don't like X."
"Ikke jeg heller"
In comparisons, it means "rather":
"Jeg vil heller ha X enn X."
"I would rather have X than X."
NB: "heller" is the comparative form of "rather", and Norwegian also has a superlative form, "helst". So, when you're not comparing one thing to another, but saying that you'd rather/prefer to have X, then you'll often use "helst".
"Jeg vil helst ha kaffe."
"I would prefer coffee."
I get it now. Tusen takk, I appreciate your help! ^^ :) Ha en hyggelig dag!