Spanish language Q&A Forum

When would be appropriate to use formal and informal second person?

For context, I'm planning to speak mostly with my Mexican family members, many that I'll be meeting for the first time or for the first time in many years. Most of them are older than me (I have one younger brother who speaks mostly Spanish). Would it be better to speak formally to the older family members, or informally because they're family?

Would you start formally with strangers (esp. those who are older) and then at some point be able to drop formality as you become closer to people, or as they express to you that it's okay (similar to Korean and Japanese)?

Hi!! I would like to ask a question. So I'm doing a presentation in Spanish and i want to use the present perfect subjunctive in the last sentence. I want to say "I hope you enjoyed my presentation". The dictionary says that it's "espero que les haya gustado mi precentación", but isn't "haya" the verb form for "usted" and not "ustedes"? Several people are going to listen to me, so I want to address them all. What do I say? Is it "espero que les hayán gustado mi precentación" or "espero que les hayáis gustado mi precentación", or is it the sentence I wrote earlier?

Thank you in advance!!

Hola,
Me confunde mucho el uso de ¨ser¨y ¨estar¨ en la voz pasiva. ¿Podrían ustedes aclararlo?
Gracias.