My OH's family are 100% toxic I have nothing to do with them at all and dont give a shit what they think about me. After 2 efforts when I discovered what they were really like I made it clear Im not involved in their silly passive-aggressive games, and thats that. He doesnt bother me about it, however. In your case OP your husband is gaslighting you by causing issues, then sitting back letting you take the blame, then as the blue-eyed boy he can go running back to his family whilst you are left out in the cold as the 'bad one'.
I wonder if his family have always picked on him, if he's always been the family scapegoat? hence constantly craving their attention & approval? This can result in always wanting others (not family members) around to blame, and deflect flak from him. Causing scenarios and then running back to family who wont see this as him being loyal at all, theyll just kick him again. After all their treatment of you is directed at him, at spoiling his relationship. On the other hand it could be that he is just like them but his passive aggression is in a more underhand way so its not so apparent to you. Again, could be to do with upbringing. Not suggesting I am right, but do you think it could be a possible scenario?
Why did you even get involved in present buying, as it sounds as if you know what theyre like? why & how do they know all about your financial arrangements? & by the way Id have told texted his sister right back, told her to piss right off and never contact me again. She ran to mummy to say you'd threatened her?! What kind of family are they? 
You are only involved with this family via your DH, so hard as it is dont let them get under your skin. I hope you have your own family, and that they are decent people? Sadly what will cause you an issue now is, your DHs complete disloyalty in maintaining strong family ties whilst knowing theyve shunned you. He isnt a child, he has left home and formed a relationship with you. His 1st loyalty should be to you. The dad being a priest doesnt mean he wont be a horrible person by the way, anyone can appear to be good on the outside yet be rotten inside. They must all be very miserable people if they need to project this kind of negativity as a family.
Stop looking at the FB posts. Sit down with your DH and get this sorted out once and for all - if you can as sorry, he doesnt sound to have any sense. 3 days away from you, to spend with his family? Dont they all have partners, who will also be there? Are you going to sit at home in misery whilst he's gone? If at all possible, take yourself away somewhere if he insists on going. Anyway, complete disloyalty and I hope you find a way to get through to him that, he is being ridiculous and needs to grow up.