Market Select -Yikes

All of a sudden, mt two brothers Jazz and Fatty about 8 and 6 months are only wanting Market Select by Meo Mix for wet food! They walk away from Pet Guard, Nutro wet (chicken and Tuna). Fatty will eat some chicken high grades, but Jazz will only devour Solid gold Tuna. It took us weeks to get off fancy feast...Jazz's tummy couldn't handle it. As for dry food, we freed feed a combo of Nutro Sensitive Kitten Food and Pro Plan and other ASSORTED KIBBLES. Mom puts them in a storage tub and mixes them up and then into those feeders that go all day, for when she is away. We are not hungry. But I am new and only fifteen weeks...what should I eat. Mom can't get the energy to chop chiken, put Tuna water in it and rice. She did that with the boys and it only worked for so long. Occasionally they will eat Natures Balance or california Natural...most of the time they like this cheap stuff. I'm aPerisan...I go in the litter box,* but no-one has taught me to cover it up yet.....What to do?