Yes, this is a very explicit statement about Fanny's 'engaged heart'. Prior to this we have had indications in the strength of Fanny's jealousy of Mary. but these have been ambiguous. Her jealousy could have been aroused because she is so very dependent on Edmunds goodwill and affection but her feelings for him might still have been those of loving sisterly friendship. The 'engaged heart' statement makes it clear that this is not so.

If Fanny's heart was not engaged with Edmund I think she would be extremely vulnerable to the attentions of a man like Henry. She has so few sources of affection and encouragement in her life, and she is so tender hearted, even towards people who do little or nothing to deserve her affection.

Fanny has more than her love for Edmund to protect her from Henry - she has disliked him from the start, has seen the impropriety of his behaviour with Maria and Julia, and thinks he has a "corrupted mind". Strong stuff indeed - but JA tells us that Henry might overcome it. I was so repelled by observing the way he operated with Maria that I'm not at all eager to see him try.