'You have no idea how your kindness and generosity has helped us.
With deepest gratitude, we remain
Affectionately yours
Flopsy and Mopsy'
As they are inlaws, it is appropriate to apply the term 'affectionate.' Had they been outside the family structure, then 'faithfully' or 'sincerely' would be appropriate.
I commend you in your recognition of the importance of writing. Sadly, many simply wouldn't...or possibly even worse....do so in the form of an email.
I remain
in His service
Fr. Bill