Or, more simply, 'Incredibiliter pulchra es, meae deliciae'.

I've employed an h in 'pulchra', as inserting an h after c has been the standard orthography in this word since the late 1st century CE. But in the very best periods of Latinity (i.e., the Ciceronian and Augustan Ages, c.80 BCE-c.17 CE), the standard orthography omitted the h. Hence, 'pulcra' in Cicero or Livy, but 'pulchra' in the Plinies or Tacitus.

'Meae deliciae' means literally 'my delight', 'my charm', 'my allurement', etc. It is vocative plural in form but singular in meaning. The singular, especially in this sense, is extremely rare in Latin of all periods.

'Incredibilem in modum' is rather more colloquial than 'incredibiliter'. Hence, this being a way of closing a letter of presumably informal content, I deemed it preferable to the more literary and formal 'incredibiliter' ('incredibly').