Your father remarrying does not mean that he is replacing memories of your mother nor does it imply that you have to forget your biological mother. However, this concern and ambivalence is extremely normal. Your father’s extreme loneliness and depression can create problems and needs to be addressed. Also all of you should find some way of discussing the issue and dealing with the grief of losing a loved one, which seems to have piled up. I would suggest that you speak to your father about your concern and family counseling should be considered.