I'm going to tell my sister she deserved to get fired because she spent all her time at work on Instagram
Did she even get anything done at work?
Nothing. It's her fault for spending too much time at work on social media
The best thing you can do is be a sounding board for when they are ready to talk. Telling her she deserved to lose her job would be counter productive of trying to get her back on her feet.
You don't know my sister. She's nothing but a lazy bitch.
She must not be too lazy if she had a job. You should try to be more supportive of her. She is probably sad as it is that she lost her job, there is no reason to make it worse for her.