This is why I turned off random flirting between sims that have no romantic relationship via MCCC. That way I don't have to worry about existing relationships. I mostly get the hurt sentiment between siblings or between children and adults. Besides apologizing the best thing to do is give it time. It also helps to keep the hurt sim away from the sim that caused the sentiment for a while so he won't keep getting hurt. Once the sentiment is gone everything should be fine again.