19-year-old sister refuses to let her 15-year-old brother take her artwork and claim it as his own: 'My brother scratched out my initial and write his own, as well as writing his name on the back of the canvas.'

If they have the same art teacher, she's going to see right through this.

As the oldest child, I never considered going through my siblings' old assignments, making a couple of changes, and turning them in as if they were my own. I don't think my younger sister could have done that to me, even if she wanted to, because by the time we were in high school, most of our papers were submitted online rather than on paper. Moreover, she probably didn't want to copy my papers anyway. They were not worth writing home about, and she has way too much integrity to be engaging in something like that. 

One person who was willing and eager to copy their older siblings' past work was Greg Heffley from Diary of a Wimpy Kid. He procrastinated on writing his history paper about the 1900s, and he paid his older brother, Rodrick, five figures in Mom Bucks (don't ask) for access to one of his middle school history papers. Greg was convinced that their dad had typed up all of Rodrick's history papers, making them of quality, but it was clear that wasn't the case. Rodrick gave Greg a poem titled "A Hundred Years Ago," in which he contemplated whether dinosaurs roamed the earth a hundred years ago, rendering it unusable for plagiarizing. 

Aside from the obvious ethical issues, using your older sibling's old assignments is usually more work than it's worth. As with most plagiarism, it takes as much effort to just write something from scratch as it does to get away with it.