Demons wouldn't dare touch her, even without Lilith's help, everything burned didn't it. Everything burned. But she'd learn whatever this world had to teach, if she had demons to help her then she'd need to know their strengths and weaknesses. They'd be useful to her in her battle against the Halliwells, getting her sister back, and she'd be blameless. Because if it didn't look obviously her then of course Billie would want to believe the very best of her. She wanted a family too. Wanted her sister back. Just in a different way. The right way
"I'll figure out how to surrvive here. You'll help me won't you, even if the Halliwells never accept me I'll have you"
Of course Wyatt would tell the sisters, explain how she'd taken his power, used him to summon the Hollow, all of it. They'd try and take her sister away again and this time, they wouldn't get away wih it, this time they wouldn't get a chance.