Is it wrong for 2 people to have feelings for each other even though one of them is already in a relationship?

Yes and no... I have been in this situation and it's really tough. Most people will say the person who is in the relationship is a bad person and is cheating however if you like two people at the same time then there's nothing wrong with that, it just depends on how they act on it.

The person in the relationship would be the one who decides what happens though, as they will either need to end the relationship or tell the other person to move on, both of which are extremely hard things to do as n a way everyone is getting hurt.

It wrong and right. If you both have feelings for each other then you should be together. Since one of you are in a relationship, than that person should break up with the person that they are with so that you aren't cheating. Feelings are feelings and you can't help how you feel. If you are going to be in a relationship together, do it right and end the other relationship first so that no one is cheating and no one ends up getting hurt too much. Hope this helps! Good Luck!!!