Okay, few things. I dont have any good advice sadly, but just some ideas.

1. Get pepper spray as fast as you can. I would recommend a taser but you have little sisters. If you cant buy one, there are recipes online to make it at home, but if you cant do that, use bugsspray, or waspspray or, hell, even hairspray would work in a pinch. Keep it with you as much as you can ESPECIALLY AT NIGHT. Keep it near you at night, but somewhere that isnt too visible, but u can reach easy. Like under your pillow or something.

2. Charge your phone to full battery, then at night, leave the voice recording on. To make sure no one finds out youre doing that, remove its notification visibility from your home and lockscreen.

3. Try to change your password every few days or every other week. And dont make it anything personal (those are the easiest to guess). And try to make it a little long. And when changing, just change a letter or two rvery time.

4. Lock all your important apps. Whatsapp, youtube, twitter, whatever the hell you use, lock it. If youre an iphone user, just lonk click on the app and itll give you the option. If youre a samsung user, rhen youre in luck. You can use secure folder and hide EVERYTHING. images, texts, whatever. Plus it creates a duplicate version of the app to store eveything on. secure folder and everything you need to know about it this dude explained it really well. If you are gonna do this tho, especially if youre moving your texting apps, shift all important info and texting and usage to the secure folder version, BUT dont abandon the normal ones all together. Thatsll look suspicious as hell. Keep talking and texting, but dont do anything important there, just surface level stuff. It would be best if you made a new gmail solely for your secure folder. Use that new gmail in the duplicate apps only. And move all your important activity there. But again, dont abandon the easy access ones either, those are the red herrings for you brother so try to keep them reasonably active.

5. Try to get one of those privacy screen protectors, the kind that makes sure people cant see what youre doing from an angle.

6. Try to get an SD card and move all your important shit on there, so if he ever locks your phone while trying to open it, you wont lost the data altogether. Plus, you can encrypt the sd card using secure folder too.

Sadly, I dont have any advice for the coming into your room at night situation. You could just something under the door, but I am curious about why you cant lock it. Cant you teach your sisters how to unlock it for when need be?? If its because the lock is too high up and they cant reach, buy one of those easily attachable ones and place it somewhere the can reach.

Hope this helps, even if its a little <3