How does someone get lice?

I moved out from my dads and in with my mom. And my mom is married and has 2 new stepdaughters. Well, one of them asked me to do their hair and as I was I saw she had lice. And eggs. And I told my mom and she looked and the other one had it too. So I told my brother and he asked me to look and see if he had them and he did. And so did his girlfriend. I told my mom and she got really pissed off because she said everytime they go to their moms they come back with lice and then everyone else would get them. I was really scared I had them so I asked her to look and she said she didn't see anything. And just to be sure I treated my hair and combed it with the comb they give you and I didn't have anything. I remember when I was little I had them once when I would stay at my bestfriends house who lived next door so we were always at each others house. And I haven't gotten them since. And I know I've been around them. Maybe its because I'm always washing my hair and doing my hair everyday? I have no clue how I didn't. My mom said you get them from living in an un-clean environment. I told my dad and he was like just keep all your pillows and covers to yourself. And wash them. And not to use anyone elses hairbrush and not to let them use mine. I know lice isn't a big deal but I'm 18 and haven't had them since I was like 8. So its nasty just to think of bugs crawling in your hair and laying eggs and them hatching its so.. disgusting.