What I would do in this situation is announce to the whole family that NO ONE except you are to feed the cat. Tell them DO NOT feed her, and you will do it yourself. That way you can monitor EXACTLY how much and what she has been eating and won't have to worry or wonder if your other family members have been sneaking her extra behind your back.

BUT if you do and the diet isn't helping, I would bring her in to the vet for a checkup. It could be a medical problem that diet won't help.

PS: your font color is VERY hard to read I had to highlight the text, hehe.