How to correct a spoil teen?

I'm the sister (21) who takes part in parenting my siblings with my mother.My mother continuously spoiled one of my siblings since a young age... giving into tantrums and buying excessive toys (weekly) despite us having very little money for that.I told her over and over again to not do that or else she would be reinforcing his bad behavior but she would snap at me telling me not to tell her how to parent her kids and that I will know what it's like when I'm a parent myself.Well unfortunately (for her) I am planning to be childfree later in life because despite telling me to step back every time she spoils her kids, whenever they're causing trouble I was the one who was called to correct/guide/discipline them. Made me out to be the bad guy. I was the one that told them to stand in the corner, explain to them why their behavior was unacceptable and come up with fitting (but nonviolent) punishments like writing out sentences or whatever.So they ended up respecting me more and she would tell me to come home/downstairs whenever they're acting up.Recently the oldest of my younger siblings have been acting up more and more. He has gotten to the age where he realizes that my mom doesn't have any control. Talk to her disrespectfully and cussing in front of her. I lost control because to him I am too strict (which is not true. I am very reasonable but compare to his mom who BEGS him and/or gives up of course I seem like the strict one).He leaves whenever he wants and never answers the phone unless he needs something. Despite us having very little money he still buys expensive things. Doesn't give a shit about his family.One example: Hurricane Harvey warning this weekend. Street is wet with rain and no stores are open but he wants to eat outside despite there being food in the house already. Since the stores are closed he is 'forced' to eat home cooked food and start throwing tantrums (beating on furnitures and cussing at his youngest siblings and getting an attitude with my mom). I have given up on talking sense to him because he just answers: I don't care.Mom wants to send him to military school or boarding school, but I'm thinking maybe Boy Scouts? I'm worried they might try to push religion on them because we aren't very religious. I'm also thinking about going to family therapy but it won't be for a few weeks since my mom is getting surgery soon and will be in the hospital and then recovering.Please give me some advice. via /r/Parenting http://ift.tt/2xCJZoO