I'm so sorry that is has taken me forever to read your fic; I still have at least five more to go, but since I couldn't vote this year, I wanted to take my time and really savor them all instead of having to rush through them to get my vote in.
I know lots of people avoid Mpreg, but I think it's got such potential, and if done right, like you did here, it's wonderful. Only you could write a scene (the last one, on Samhain) that is funny and sad at the same time. You captured Snape perfectly throughout the entire fic, but the last scene is incredible.
And twins! I made an MP3 file so I could listen to the fic while I walk and commute. I had just pulled in the driveway and had to sit there in the car, listening to the end.
Great job, and it plays well as an audio file, so that says something about the clarity of your writing too.