If your sink is big enough, I'd just wash her in the sink for a while. Put a nonslip mat on the bottom, and stay right there with her. She'll grow out of it, especially after seeing her sibling having fun in the tub.
If your sink is big enough, I'd just wash her in the sink for a while. Put a nonslip mat on the bottom, and stay right there with her. She'll grow out of it, especially after seeing her sibling having fun in the tub.