I have no experience with blood pythons.

I've been reading and it seems most males get upwards of 7lbs. So over 3000g. Ive read of massive females reaching 40lbs, although the website does state that bloods of that size are usually obese.

Taking into mind how thick bodied they are, I wouldn't compare them to a ball python but more like a miniature BCI female. I have a 7' female, weight unknown, and she has left marks on my arms from when she used my arm just to wrap around and steady herself.

As long as you are closely monitoring the snake when it is out of its enclosure and around your younger sibling, I wouldn't be too worried. Although if it was to become frightened, or you were having it wrapped around your neck, I could see some problems.

Just be sure to keep the enclosure locked, and inaccessible by your sister if and when you are not around.