Is my sister using drugs again?

I don't have much to do with drugs but I need some advice.
I live with my sisters. We all share a house together my middle sister has just moved in with us after being on the other side of the country.
Now in the past she was a heavy drug user she has been quite erratic in the past but she says she's been clean for years now.
She's having some erratic behaviors again. She can be happy one day and then angry the next. All can be going well for a week then she flips she lost it at me once and she ended up pulling me by my hair to the ground. Then we go on again all normal. Then one day she comes home so happy her life is amazing she says. Then I say the wrong thing and she's tipping her coffee all over my uni work. I'm a mess at this stage, screaming at her so she throws a plate at me.
Well today we wake up and she is okay my mum comes over and they go out. I take my daughter out for lunch. We get home and then my sister comes home. She's yelling at me to move my car out of her parking spot so I do so. Then she is laughing with my daughter playing with her. Then she takes my mum aside who's ready to go back to hers and breaks down I mean sobbing.
The night goes on my mum stays for dinner.
My daughter is then lying on the couch she's only 3 and a half. And she wants to sit down next to her my daughter tells her to go away she lying down. She grabs her feet and lifts them up my daughter gets up set and she has a go at her saying she needs to respect her elders she has no right to boss her around and calls her a little bitch. I stood in I was so mad no one calls my daughter a little bitch. She kept saying all this nasty stuff to her saying she was going to be a bully and you mum is going to let you be because she is sitting there with you while you cry and not telling you to respect your elders. I told her to get out.
Next minute she's leaving the house she then again comes home and she goes to bed and goes "thanks for saying good night to me". Shes in her room sobbing my mum still here, she goes in and she goes off she's mad saying the world hates her that she needs to die. She runs out of the house and gets in her car and is reving in then takes off and sends my mum a message saying a storm is coming.
Last night she went some where when we were all in bed then came home 10 minutes later. Was that to get some drugs?
She's been like this for probably 3 months now. Just up and down crying over the smallest things like not having a jar to put pasta in or laughing at the weirdest things. Then saying every one is out to get her.
I don't know what to do. Is she on drugs again, or is this just because they have messed with her pretty bad.
How do we get her help or does she have to come and ask for help.