A perfect sentence. However, just because of common usage, I think you have two better options: 1. Thank you so much, but I can't make it due it to family reasons. 2. Thank you so much, but I won't be able to make it due to family reasons. 'won't make it' is not incorrect, but it is more common to say the two things I wrote in response to an invitation.

The sentence is ok, but it's a little odd because it's a mixture of formal and informal language. 'Make it' is informal and casual, used for spoken language. 'Due to family reasons' is formal and official-sounding, and would be more appropriate for written language. You need to decide what the context is, and then rework the sentence appropriately - either informal spoken language, or more formal written language.