years back, a friend told me that vriska's mind control only worked on the weaker-minded (like tavros), and definitely not terezi. is this true? or did terezi's mind powers result in her immunity, if there even is an immunity. (im guessing aranea's is much stronger due to her age)

Terezi and Aradia are both immune. The fact Aradia’s immune means it’s not caste-related, nor is it Aspect-related, but it’s highly likely lower castes are just usually more mentally weak.