The main theme seems to be madness: the madness of father-figure Cousin Ambrose's grandfather, father, and possibly Ambrose himself; the mental decline of the main character, who is also the narrator, Philip. And of course, the supposed money-mad Cousin Rachel who 'possibly' did everything in her power to dupe the immature and naive Philip, as well as the incredibly warm-hearted second husband, Ambrose. 
 Cousin Rachel is certainly no innocent; she seems to deliberately lead young Philip on until she got what she wanted from him, according to our very unreliable narrator (reminds me of Charlotte Bronte's Lucy Snow and her descent into madness). We can never really know if all he speculated was true. 
 As for Philip, his clearly dark obsession with Cousin Rachel, to the point of imagining themselves married, not caring about his life or the land he so clearly loved in the beginning of the story, is alarming, to say the least. 
 Cousin Rachel is clearly an emotional victim, to both Ambrose and Philip, but a victimization that she actively sought for in the pursuit of economic stability. On the other hand, she also makes them both victims of her feminine wiles, which they are clearly not used to, so they too are victims in turn. 
 Of course, all three pay dearly and there is clearly no return, not even for Philip.