Dont let them intimidate you. I am sure your family is not the first to go thru this so maybe you can get some legal facts from the internet-know your rights!! Never give up your animals no matter what they say. I read something where this happened to a person and they gave up their dog voluntarily to the SPCA for the SPCA vet to evaulate health and they never got the dog back. The SPCA does not have to be allowed inside buildings unless they have a warrant. They need to have just cause to get that warrant, no judge is going to provide one on a whim. They issue cruelty violations for lack of shelter, water/food, vet care, and sanitary conditions. SO YOUR UNCLE MUST HAVE THESE TAKEN CARE OF AT ALL TIMES if he is getting surprise visits. If he has dogs outside he must have a dog house, and best one of those insulated igloo ones. They may even want one for each dog. Sanitary issues means no poop or matted piles of dog hair, no strong odors...anything that could be germy. This also includes fencing in disrepair, like bent wires on kennels or anything that could injure a dog.
MAKE SURE ANY ANIMAL THAT REQUIRES VACCINATIONS IS UP TO DATE and paperwork handy if you have to show it. Licensed, too, but that is probably a Dept of Ag thing and not SPCA but it never hurts to overachieve on regulations. Sorry..this situation stinks but get all your ducks in a row and the SPCA not being able to harass your Uncle the best revenge.