I'm going to tell my sister I am worried about her being pregnant because I'm not sure she is financially stable enough
What makes you think she isn't financially stable enough?
She has almost no money in savings and she's constantly jumping from job to job.
Being concerned for your sibling and their future child is a really normal and good thing to do. And you're right, she should be financially stable if she plans on having a kid. It's a very delicate subject so try to be careful when you talk to her about it.
I'm also going to make fun of her.
That isn't helpful at all. You can be concerned without making fun of her. All making fun of her financial status will do is make her upset.