What is the ruling on preventing one's parents from visiting friends, and is my informing them of this considered backbiting or lying, and should I evade answering? And what should the maternal aunt do if her husband prevents her from visiting his sisters, and would informing her of this cause discord and hatred?

If you fear that informing her about your parents' prohibition or your aunt's husband's prohibition of her would stir up animosity or enmity, then do not tell her. Instead, use allusive speech (al-ma'arid) to convey the truth without lying. However, if no harm results from your informing her, then there is no objection to it, and it is not forbidden backbiting (namimah (definition)). Backbiting is revealing what is disliked to be revealed.