Looking really good so far.
The only thing I have a bit of trouble understanding is that last sentence, you say it's a blood ward (so no Hermione), but Remus can join them? As far as I remember there's at least no official relationship in-between Remus and Tonks in the current time in the timeline? I guess Sirius could just have added him to the wards.
Anyway great start, really looking forward to more.