It’s phrased in a very “the lady doth protest too much” kind of way, because she hears someone mention the concept of trading in human flesh and immediately assumes it’s a jab at her brother-in-law - which suggests that her brother-in-law (not her husband, father, or sibling, very specifically this one person she is connected to) was involved in the slave trade before abolition.
It’s like Ivanka Trump hearing someone mention the border wall and turning around to say “My father is the least racist person you’ll ever meet!”
Fanny Price’s one mention of the slave-trade is met with dead silence. She asks Edmund if he heard her ask her uncle about the slave trade, he says that he had hoped she would ask him more questions, and she responds that she would have if her question hadn’t been met with dead silence. There are several interpretations to this, but honestly my feeling is that Fanny asked a question that she thought was apt and it went over like a lead balloon.
Fanny is a *fascinating* character because she is so completely broken by her family that you have to approach her as an unreliable narrator. Fanny looks at the world like a kicked dog and it makes Mansfield Park absolute torture to read sometimes, and part of that is in her devotion and generosity toward her *awful* relatives.
I would actually argue that Sir Thomas Bertram is one of the worst of several very bad men in Mansfield Park. When, after years of abuse at the hands of the Bertram family and constant reminders of how much she owes them for allowing her into their home to be abused, she refuses to marry Henry Crawford and tells her uncle that she could not like him enough to marry him, he criticizes her for essentially talking back, saying that he thought she was free of the independence of spirit that infected young women these days. (This paper is good overall, but look specifically at pages 10-11 for how poorly the Bertrams generally and Sir Thomas Bertram in particular treated Fanny). My general takeaway is that everyone at Mansfield Park is terrible enough to deserve each other and that Fanny and Edmund are lucky to get away and be happy and poor together.
There are plenty of people who see Austen as an apologist for slavery, there are plenty who see her as an abolitionist. I don’t think that Austen was an abolitionist in an activist sense, I do think that she was likely very sympathetic to abolition.
Reading through Austen analysis is interesting because you stumble across a lot of people who have no idea what is going on. Austen was relatively subtle in her own time, and now seems so subtle that she is opaque. If you want to get a feel for that, look at the Jane Austen Fandom Wiki, where nobody who has spent more than an afternoon with a Jane Austen book has ever written a description of any of the characters. Their description of Sir Thomas is so anodyne that it becomes revisionist.
Genuinely, there is a lot of context that modern readers have lost when reading Austen. There are a lot of references and allusions and turns of phrase that fly right over our heads unless we spend a lot of time catching up with them. It’s a thing that happens in literature as time passes, it’s why Shakespeare nerds and Chaucer nerds are slightly crazy - they know all these excellent in-jokes and they can’t explain them to normies without writing a dissertation. Austen is very similar, and because her books are often presented as comedies of manners it is easy to overlook the fact that nearly all of them also contain tremendous tragedy.