Brother-in-law accuses me of forgery at the wake but his lawyer letter has the wrong name

I'm in Queensland, Australia. At my mother's funeral wake last week, my brother-in-law Brendan Walsh cornered me in the kitchen and loudly accused me of forging her will to cut him out of the estate. He shoved a folded lawyer letter into my hand, said "you have seven days to hand over the keys," and stormed off. Aunty Janelle tried to calm everyone, but the damage was done.

When I finally read the letter in the car on the way home, I noticed the top line: "To Liam Chen." My brother Liam. Not me. The letter was from Brendan's lawyer, demanding that Liam surrender the house keys within 7 days or face a claim for trespass. It referenced a "dispute over the validity of the will" and said Brendan had instructed them to take possession of the property as "the rightful beneficiary."

I called Liam that night. He told me he and Brendan had a falling out six months ago over a $15,000 loan Brendan gave him for his small business. Liam said he'd been paying it back in cash, but Brendan started gaslighting him, denying the loan ever existed, and then cut off all contact. Liam said Brendan had been sending him threatening texts for weeks and that this lawyer letter was just the latest escalation. He'd already hired a solicitor.

I hired a solicitor myself the next day. We sent a cease-and-desist letter to Brendan's lawyer, attaching a copy of the will - it was signed in front of two independent witnesses at the hospice, exactly as required under Queensland succession law. We also pointed out that the demand letter was misaddressed to Liam, not me. My solicitor made clear that Brendan had no standing to demand keys from either of us, since the will appoints me as executor.

Aunty Janelle called a family meeting at her place yesterday. Brendan showed up and, under pressure, admitted he "might have" sent the letter to the wrong person - he said he'd meant to send it to Liam but grabbed the wrong envelope. But he still insists the will is a forgery and says he'll challenge it in Supreme Court. He's claiming I "unduly influenced" Mum in her final weeks, though I was barely there - I live three hours away.

What can I do here? Should I talk to a lawyer about a costs order if Brendan's claim is frivolous? Can he force a sale of the house while the will is being challenged?