If you have silkies, it's not the best thing to let them free range without supervision. They can't fly to get away, and if they have large crests they can't see something coming for them. It's just a recipe for disaster.
The unfortunate thing about cats is that they are, in many jurisdictions, considered "free roaming", which means cat owners are not bound to keep them in their own yards. You can try talking to your neighbors, but my experience has been that it never helps. Generally, your only choice is to trap the cat while it's on your own property and take it to the local Animal Control. The owner may need to pay a fine to get it out and that may encourage them to keep their cat inside, but probably not. So you need a secure pen where your birds can be safe during the day.
And . . . so sorry for your loss.