I fought my younger brother when he flat out told me that whatever he's doing at the moment is much more important than any problems that are not his and that it is too much of a hassle to even "open his room's door for me to pick up a few tools from his room" (said tools are mine, he borrowed it a week ago and is not using them at the moment).
At that day, he was making electric piano music for his Youtube channel, while my task was fixing the printer with a blown electric motor.
I got my grandmother to help settle the dispute, saying along the lines of: "Your younger brother should help older brother on ALL the chores and activities and such!" But his anger kept him from understanding the situation.
Earlier on the same day, I also got angry at him for not giving to me the phone, instead he uses it to make memes, as odd as it may seems. I told him to bring our phone ("our" as we share a single phone) to perform calculations on my Excel app on-the-fly to see where we could save money, while getting all of the desired items for everyone. We were celebrating my cousin's birthday, along with his older brother, younger sister and my grandmother, so budget is important here.
I am the oldest brother, aged 22. I am tasked to be a good and responsible adult to my grandmother of 70, my cousins of 5, 7 and 8, and my younger brother of 16. Not having parents doesn't help things, and the "ALL of the chores and activities and such" is I think just too much as my grandmother is literal on her advices. I only ask for the things that are within his capacity of doing; he's good at music and internet, not on electrical and electronics stuff.
The question is:
"How to convince my younger brother to stop what he's doing at the moment to help me and my grandmother at things we're supposed to do and explain to my family in such a way that everyone understands it and sticks to them, as to prevent things like this from happening again?"
What I've tried
Please understand the context of my situation here. I have already done the "ask your family about it" thing and it doesn't work as my grandmother already had tried. My cousin's mom is "always away" while their dad's currently serving time in jail. I got nothing in my experience to handle this and my friends have nothing on it too. So how could I handle this situation?