You are under no illusions as to what it means to be turning into a vampire. While many who changed into creatures were kept around as slaves or servants, or even merely driven out of human society, there were certain species that were mercilessly put down either by state or temple as soon as they were found. These included demons, feral creatures...and vampires. The reason being that usually these creatures eventually gave in to their violent or murderous urges and began killing humans. And now, you were one of them. You had killed.
You sit there slumped against the wall for some time, you cannot say how long. Eventually the obese body in your arms begins to grow cold. Thankfully, the alleyway remains deserted. Finally, you come to your senses. If someone discovers what you are, you are as good as dead. However, that is only if someone finds out. While you are not sure how long you can keep your true nature hidden, for your siblings' sake you have to do so until you can come up with some sort of plan.
Recalling that there is an abandoned well nearby, you get to your feet and decide to heave the body over to its location. However, you find that the corpse seems to be less weighty than you assumed. In fact, you are able to lift it rather easily, given its sheer size compared to yours. Is this another change because you are a vampire? You have no time to think it over, but hurriedly heft the body over to the boarded-over well, then quickly pull back the wood planks nailed over the opening.
Just before you pick up the corpse to drop it in the well, you hesitate. Not because it would be desecrating a body. That boat had long sailed. Instead, you reach into the fancy jacket and feel around, eventually withdrawing an embroidered purse. Most people would be horrified at robbing the dead, but if you are to keep you and your siblings alive, you need to make use of everything you can. Dumping the contents of the purse into your hand, you shove the coins into your pocket and return the purse to the body's jacket. Finally, you dump the dead body down the well shaft and return the wood planks to their position.
With a little luck, no one will find the body until it rots enough to release a god-awful stench. By that point, hopefully the evidence of your crime, including the puncture marks from your fangs, will have decomposed away. You are somewhat surprised how calm and methodical you are, but perhaps it is just shock.
Returning directly home, you catch a glimpse of yourself as you pass the broken mirror and stop still in your tracks. No longer a dull ashy color, your hair is now a pure, unmistakable white. Dumbfounded, you are at a loss, but you still don't know what to do next. However, you cannot let your siblings see this. Tying up your hair in your head kerchief, you use a bit of soot and oil to create a sludge to cover your eyebrows and eyelashes, also white. You finally nod off into a fitful sleep a few hours before sunrise.