Can I get EEA FP as non-EEA national if Brother is Dual EEA and UK citizen?

I hope I can get some advice as there seem to be no legislation or guidance that I can find on my particular issue.
My brother is married to a Portuguese citizen for 20 years now and he was South African, but got UK citizenship 15 years ago. I know that you used to be able to apply for an EEA FP as a non-EEA national if your family member's spouse is an EEA national, but they removed that option a few years ago. Lawyers knew of this but advised me wrong as I could've got an EEA FP back then. Now I no longer can.

Due to Brexit, my brother is applying for Portuguese citizenship. Now, if he is a duel national, does that qualify me to apply undur his EEA nationality? There doesn't seem to be legislation to clarify this.

I understand the principle now is that dual nationals, i.e. British + another/EU nationality, are treated as British for immigration purposes. That means they cannot sponsor under EU regulations, only under UK immigration rules, with its strict requirements, minimum earnings, etc.

Ok thank you.