My kitties are disappearing

I inherited 3 feral cats with my new house. They were a gray female named Sassy, a brown tabby, Mamma Cat and her little tom, Jazz. Then Sassy brought up her babies about a month after we moved in, two kittens I named Bella Sera (a gray tipped showshoe siamese girl) and a brown tabby boy called Simon. Then Mamma Cat had babies again on August 23rd, four of them, Bootiful (female solid black long hair), Tahlly (female long haired tabby), Walker (steel gray male) and LiLi (long haired gray female). Tahlly and Walker disappeared when they were about 12 weeks old. I gave away Boo and LiLi.

Then in January Sassy just quit showing up to eat. A couple of weeks after that Mamma Kitty disappeared. 10 days ago Jazz quit showing up to eat (and he was actually coming into the house on occasion and would let me pick him up.) I haven't seen Simon since yesterday afternoon and Sera is calling and calling, so I think he's gone, too. They were littermates and practically inseparable.

I spent a lot of money on Sera getting her fixed, shots, fleaed, etc. She's the only one outside how and I hope she is safe. Even though she lets me pet her and pick her up, she is still too feral to come in. I'm worried.