Translation:My sister has taken my horse which I did not like.
"Som" can only refer to the horse here, so it means you didn't like the horse. There's no ambiguity in the Danish sentence, unlike in the English. If you wanted to say you didn't like your sister taking it, then you'd have to say something like "og det kunne jeg ikke lide (at hun gjorde)".
I'd like to hear from a Danish native speaker: If it were about the action of taking the horse and the other person not liking that, would "hvad" really be correct, or not rather "hvilket"??
My sister took the horse. I did not like her action.
What would that be in Danish?
"Min søster har taget min hest - hvilket jeg ikke kunne lide." I guess, "hvad" would be wrong here, but I am not sure... In German, one would have to use "was" and in English "which" in that case, though many Germans would unfortunately also use "was" in the other case: "...the horse that..." ("das Pferd, was" - which is clearly wrong).