It is confusing, certainly, but I am grateful to you for having shared it. My understanding is that there is only a single timeline, but that time travel, or situations such as we find ourselves in here, cause alterations in it, which can have all manner of consequences. Time travel is tightly restricted in wizarding Britain for that reason.
Yet the undeniable fact remains that you changed the timeline, and you and your father are both here. It may be a bubbling effect of some kind. I shan't pretend I have the expertise to explain it. I know that when I was sent back before, I remembered things, but vaguely, as if it were half a dream.
The fact is that my nephew Sirius, who died as a little boy, is here and very much alive, and there may be a way to keep him that way. To cure the fever before he even contracts it. I am beginning to think it is best to try, even if we cannot be certain of the results.