Why did Martha say "You're gonna be a terrible mother"?

Because they were gonna send her away and she said it out of anger to hurt her sister knowing they were trying to have a kid. Keep in mind martha has been psychologically damaged from the cult

While she mainly just said it to hurt Lucy, it's not completely out of nowhere. It's made clear earlier in the film that Martha can't get used to the idea of Lucy being a mother. She laughs uncontrollably when Ted tells her that they're trying to have a baby and I think she even tells Lucy to her face that her being a mother is a weird thought.

I think we're meant to remember that, in Patrick's "family", one of the things the women did is look after the baby. With that in mind, Martha's inability to see the rather aloof and unsympathetic Lucy as a mother is just another example of her inability to separate the two worlds she'd occupied: the cult and the 'real world'.

Because she's a dick!

😎