Yes, it's hard when they cry and scream all the time, but 18 month old's benefit from discipline, and if you nip the tantrums in the bud you won't be dealing with them later, and your 3 year old won't get any ideas to start imitating her. My daughter was around 18 months when she began to have blood-curdling screaming tantrums, sorry, I don't do tantrums with anybody! For the sake of your nerves, your tiredness, and with no family support team you have to do this on your own, but you can.
You are a parent who I'm guessing is trying to establish acceptable behavior so you have to be proactive. You can't assume it's a phase and hope it goes away on it's own, unless you don't mind a bigger problem to deal with and undo later. These are power struggles, and her weapon against you is her temper tantrum.
Whether she is angry, tired, frustrated, or simply didn't get her way, discipline by ignoring her, which since she's a screamer I highly recommend (I left the room after making sure she was safe, and no audience is no fun) or isolate her in a time out. I have a friend who's grandson is 17 months old, and he knows that his chair is for time-outs as he's been walking himself there for months, so 18 months is not too young.
Pick her up from behind when she's having a tantrum so if she goes to arch her back she arches into you. And watch your head, she may try to hit you with hers. Simply tell her what she did that you don't like and that she is going to her room or a time-out, then no talking until after she has calmed down. Don't argue, which prolongs the tantrum, and for some reason the silent treatment lets them know you really mean business.
Be firm and consistent, but loving and understanding with your discipline.
Keep explanations short (Because I said so,") until she is older and more verbal. When you discipline her, make a verbal separation between her and her behavior ("I love you, but I do not like it when you scream." Pick her up, hold her, or remove her from dangerous situations, of course, and I know this is hard with a thrashing child. Reassure her once the negative behavior has stopped.
Consistency each and every time she has a tantrum, in love, will teach her she can't win the power game, and give you more time to spend enjoying your girls.
God bless<3