"Cousins in love" -- "Cousins in Law"?
This does not appear to be a typo. The book has been edited and re-edited multiple times since its original publication and this would have been corrected.
It's not clear from the text whether or not he's speaking of cousins who are actually in love with one-another, or rather just engaged in a relationship/marriage that was rather common in the times and writings of Jane Austen.
From context, it seems that he's considering one of his sons falling in love with her.
An Ngram isn't particularly revealing, either.
