My brother scares me half of the time.He doesnt go out to scare me...he just does..?

Ok so I am a 13 year old girl.I have a 23 year old brother.He scares me half of the time.I don't know why.He doesnt do anything or change moods.Half the time I love talking to him because he's awesome and really fun to talk to,but the other time im positive he hates me and im scared of him even though he's still really cool.My mom is dissapointed because he never finished school and still lives with us and my step-dad has always hated him.He's pretty much the dropout guitar player,that smokes,and his parents are dissapointed in him and he scares the crap out of his little sister.Im seriously like 90% sure he hates me.I don't know why he scares me.I wish I did,but i dont.When I talk to him hes nice.I dont really think he would hurt me.Why do I feel like this? and what to do?