People you know in dreams can be literal or symbolic. Dreams are symbolic expressions of your feeling, thoughts and awarenesses about your waking life at the time of the dream. So, if not this acquaintance, who else are you sensing on a subconscious level may be upset because you are not picking up on HIS romantic interest in you?
The next part of your dream is about a family situation, given the presence of your estranged sister and your Dad. Estrangement is often hurtful, thus perhaps the sense you had that she was "trying to hurt" your Dad.
Then, all of a sudden your acquaintance friend is there to forgive and comfort you, perhaps because the issue with your sister and father is worthy of empathy and compassion, even to someone who may feel jilted by you. That acceptance and forgiveness that he shows you makes you wonder (as voiced by your mum in the dream) whether or not you do have feelings for him.
Without knowing the waking life background that generated your dream, it's hard to say who this man is or what generated your dream. If you could share that, it would be helpful in giving you a more conclusive interpretation. Given that disclaimer, my best guess is that there is a man in your life who you feel wants more of a relationship with you. You know on some level that he'd be angry that you don't share that feeling, but you also know that he'd be there for you to provide support... as in the nagging issue of your sister's estrangement and the hurt that is causing your father. The reason that you don't reply to your mother is that you aren't prepared to consider your good male friend as a romantic interest... despite the fact that your mother would approve of you having someone like him in your life to love.
Hope that helps!