I would say it isn't a job, but as it seems it takes a significant portion of your time it needs to be mentioned. If I remember right there are a number of opportunities in the essays where it may be relevant to discuss it and how it has impacted you/taught you something. While every applicant probably has chores, there is a difference between taking out the garbage, folding laundry, etc... and taking care of a younger sibling on a daily basis. Another opportunity to address it would be during your interview with your ALO, but I would take some time to think about what it has taught you, how you have grown from it, etc...