I understand your concerns, and hopefully your father will come to understand them as well. But this situation is a two-way street–and hopefully you and your brother will come to understand his feelings also.
Yes, it may look like your father has forgotten your mother–but I seriously doubt if this is the case. In fact, by considering remarriage, he is actually paying her the highest possible compliment, because he wants his life to be as happy in the future as it was in the past. If he hadn't been happily married to your mother, I seriously doubt if he would consider marrying again.
I know this is a difficult time for you and your brother; no doubt you still feel the loss of your mother very keenly. But if you truly love your father, you want what is best for him–and if it is God's will for him to remarry, then you should accept it joyfully. The Bible tells us to "Rejoice with those who rejoice" (Romans 12:15).
Pray for your father that he will seek God's will in this matter. And pray as well about your own feelings and concerns. No one will ever replace your mother–but that shouldn't keep you from welcoming this woman into your family if your father does remarry.