IIRC the whole thing has to be handwritten because they use the soul imprint of the handwriting to locate that piece of paper in the first place, but only the most recent addition to the picture counts for who is signed up.
From the compendium:
Can we trick our friends into signing up for the project?
It's about who added the latest addition. Technically you could have them just write their name next to the code and a drawing, and their name would be the latest addition.
IIRC long ago we even asked about them writing on the back of the paper and even using carbon sheets to draw on them unknowingly and they said the latter works (dont remember for the first).
Also from the compendium:
How do you feel about tricking someone into registering?
I mean I can understand why people would do it. I find it immoral but it's up to you really.
Tsuki doesnt really agree with doing this himself, but its your decision if you want to trick your sister into this or be open and discuss it with her.