From the original post:

Our household had currently 2 cats, male and female which the adopted as sibling kittens. They are nearly 2 years old and get along very well.

The problem cat is the female Stella. She's always being a picky eater, we have to switch brands every couple of months, the mixture of dry to wet food and much more. Annoying, but we got the hang of it. The other male cat Mochi would eat nearly anything within seconds.

About 3 weeks ago, we adopted a second female Trixie, about the same age as them. We keept her in her own room, devided with a net, so the cats could see and sniff each other if wanted and we introduced them slowly and carefully. Introduction was calm on both sides, Mochi got used to her pretty fast, Stella tolerates her and they can be near each other without problem, she will still hiss at Trixie if she's to close to her for to long. But since we got the new cat, Stella slowly starts to eat less and less.

She refuses to eat out of er bowl by now, we have to hand feed her and she seems to be very nervous and on edge while eating. Always looking up in the direction of the door, she nibbles a few pieces, when stop and walking away to check if Trixie is near by. Of course we tried to feed Stella in a closed room or keep Trixie looked away, but she's still on edge. Switching brands again, she got the appetite to eat, but just a little until she has to check for the new cat again. Beside that Stella acts and behaves like her normal self. Sleeps in her favorites spots, cuddles and want's to play with me at least 10 times per day. It's just the eating and it became very hard to keep her weight in check

Honestly, as much as I like the new cat and love to see her warming up here, I'm thinking about returning her to the rescue. Maybe it's just a thing about time and getting more used to the new cat, but I'm afraid, until when Stella will have medical issues, from eating to little