Sadly, this is not a unique situation but it is somewhat unusual in that your family appear to have very swiftly closed ranks against your brother.
Like yourself, I see your brother's failure to protest his innocence to his nearest and dearest of great significance. As for him denying the charges, that is only to be expected.
However, the fact you have been told SS will be involved in some capacity in his child's life would suggest that your dsis's allegations were not held to be false and you're best advised to bear that in mind at all times.
That said, first and foremost my heart goes out to your dsis and I sincerely hope that the love and care given to her by your dm, yourself, and any other close relatives you/she may have, together with counselling and therapy, will enable her to come to terms with, and move on from, the experiences that have hitherto blighted her young life.
As she is so very close in age to her/your brother's gf, it's not surprising that your dsis chose to disclose the sexual abuse she endured, presumably over some considerable period of time, precisely because his gf's pg.
Given your dsis's understandably depressed and conflicted state of mind, now is not the time draw her attention to the new arrrival. It could be that she may refer to the birth herself at some point - take your cue from her; don't seek to overburden her with information or require her to make any decisions before she is ready to do so entirely of her own volition.
Realistically, you know that you cannot be a part of this child's life at the present time without the consent and or/involvement of your brother and that is neither desirable nor does it accord with your dm's wishes.
Sexual predators are invariably manipulative; in extending a wish to see your niece/nephew or play any part in her/his life you may be seen by your brother as being the thin end of the wedge, the weakest link, and you can put money on him seeking to exploit any connection/correspondence he may have with you after the event, so to speak.
Unless and until your dsis's circumstances undergo considerable beneficial change, I strongly urge you to hold a watching brief particularly as this is unlikely to be the only dc your brother has with either this or other/future gfs.
In effect your brother been placed, rightly in my view, the other side of an iron curtain. Raise it at your peril. And know that insodoing you may be causing irreparable harm to those you love most in this world.
In the fullness of time you may find that this infant goes in search of wider biological family members when s/he reaches adulthood. Spend the coming years preparing yourself for that eventuality because, if asked, you'll be hard pessed to give an explanation for the estrangement without causing him/her to feel guilt by association.
Please take some consolation from knowing that you're not alone; many others have faced, and will face similar dilemmas. Until such time as fate decrees otherwise, save your loyalties and your desire to do right for the children and young people you know and for those closest to you.