As they are both immortal, I do believe their relationship is actually beyond what we can comprehend as mortal. I do believe that both of them hurt each other pretty deeply and destroy their bond. But What seems for us impossible to forgive in a life time, isn't the case for people that can live forever.



And yes Frisk does see them both are parents, so this might be a big prove that the connection between them isn't completely destroyed.

When time doesn't have an impact on you, who knows what could really happen in years from now. Their notion of grieving is different, because being immortal is being attached to the idea that everything around will change, except you. When this happen, when everything you know keep on changing and disappear, that might impact the very definition of yourself, and I truly connection between two immortals might be vial for them, because they are a constant in time something remains no matter what.