I am glad to see 'Brethren' as an option at several places in the new English RC Mass, where it is an alternative to 'Brothers and sisters'. I have always found the latter formula awkward when facing a congregation exclusively of one gender; so much so that I have sometimes felt moved to substitute 'Dearly beloved'.
Scoffers will probably say that 'brethren' is still awkward to say when there are only sistren present. But the point is that the archaism 'brethren' is just that little way removed from gender-specific awkwardness simply because it is an archaism. Additionally, of course, as an archaism 'brethren' is a relic of a language in which grammatically the masculine was usable as a common gender 'embracing', as shoolboys sniggeringly said, 'the female'. I regard this as a sensible example of bringing all the resources of a language, even its archaisms, to the service of Liturgy. Liturgiam authenticam rightly
envisaged the development of a liturgical dialect in which a little moderate archaism would be in place (traditional Anglicans, of course, as well as many Orthodox, think that thoroughly archaic language is thoroughly good anyway).
This is bound to raise again the question of the Orate fratres. My own view is that it was really intended as an exchange between Celebrant and those around him at the altar, not as a way of asking the entire congregation to unite itself with the offering of the Sacrifice. The EF rubric sees it like this, although of course the OF explicitly expects 'the people' to reply. I would welcome the re-emergence of the custom whereby at sung masses, if the singing is still going on, priest and altar-party get the Orate fratres and its response done among themselves without waiting for the chant to end.
We should accept that traditional liturgy has always envisaged three classes of participant: Celebrant; Sulleitourgountes, the ministers around the altar; and the People. This is particularly noticeable at two places in the Canon Romanus: Hanc igitur, where God is asked to accept the oblation 'servitutis nostrae' (a Leonine periphrasis for 'of us your servants') 'but also of your whole family'; and Unde et memores, where 'we your servants' are distinguished from 'your holy people'.In each place the fact that two groups are intended is made clear by the two phrases being linked by 'sed et'. The new translation appears deliberately to underplay this distinction.
Here, as so often, being soaked in the EF will help priests and people gradually to reappropriate what is the true sense of the texts and rituals in the OF.