No, i just wanted you to check other meanings.
Sinfully is the closest meaning here as I said before
it comes as an adverb to (they eat up people's property sinfully)
The meaning in general is: they know that they eat up other's property altough they know it's an evil thing
they know it's wrong but they don't reveal that.
The idea is, English doesn't has a lot of "in details"/literal grammar as Arabic so you can sometimes understand it by total meaning of the sentence