Oh, this is exquisite! Such a tense, atmospheric tale, so very dark yet with tiny specks of light interspersed throughout. I love the pairing; I don't think I've ever read it before, which is odd, because it makes a lot of sense. Aberforth and Severus are both torn between and tortured by the love and resentment they have for Albus. Aberforth would be the one to guess what his brother is putting Snape through. The way they understand each other without talking about what is going on, the way they both understand the necessity of secrecy, it all rings very true.
The background characters are inspired; they may be mere sketches, but the sketches are incredibly evocative. And they come together to create a rich canvas, an atmospheric backdrop in front of which Aberforth and Severus' story unfolds. Your descriptions of the Hog's Head make me think of a Rembrandt painting: a background rich in details, yet concealed behind a dark veil, with the main characters standing out thanks to the skilful use of light. The only problem is that I read it on a sunny April morning: it should be read on a gloomy November night, like a classic ghost tale.
Other light bits are the glimpses of humour: Snape reminiscing about the good old times when he would get rat arsed with Minerva is a lovely bit of characterisation for both Snape and Minerva. And I adore the goats feasting on Madame Puddifoot's daffodils.
The slow, understated understanding, Severus despair and loneliness, their affection for each other and for Albus - it's all heartbreaking. And the last line is the killer.