Fighting

Just got into a really big fight with my younger brother who I am caring for. It's really sad because today is my first day in ages that I have had off work and we were going to have a fun brother sister day but now instead we have just finished a big fighting match and we are both in our rooms sending swearing texts at each other. Admittedly I started the fight about him constantly putting me down. Every time we have a difference of opinion he puts me down and makes me out to be really stupid and gives examples why. I usually just let it go but today I am tired and stressed and I couldn't stop myself being angry about it. I know I'm not the smartest person but it genuinely hurts my feelings, especially when I have essentially given up my whole life to look after him and he can't even consider my feelings. But I hate fighting with him and I get so afraid that he's going to get angry and just decide he's going to move back into where he used to live ( far away in a house full of boys who drink and do drugs every day.) it would be the biggest backwards step for him and he knows I'm really afraid of him leaving. He has used it as a threat a few times during previous arguments. I don't know whether I should apologise or whether that will just send the message that it's okay to scoff at me and put me down all the time. I also dont know if it's just me being sensitive and emotional because I'm stressed out. Any help welcome please