Legal advice from a close relative?

Is it allowed for a close relative to act a your lawyer?
I know doctors aren’t supposed to treat close family.
Can my sister who is a lawyer be allowed to act as one for me, to write cease and desist letters to a neighbouring family. Harassment, defamation, misuse of a carriage service to harass, harassment of a senior, harassment of a person with a disability, etc. I just want to be treated like a human being not a POS. It has been happening since they moved in in 2014. I’ve tried ignoring them, not taking the bait, being calm and polite, but it got worse, now I really want to send a message and pull them up and get it to stop.
My sister lives in another state – NSW. I live in Qld. Location: Brisbane.

Any advice appreciated.