Fatty is very skinny. Any advice on fattening her up?

[Toddlerteaplease]

Over the last year Fatty has lost about a kg in weight. It originally started when she had to have wet food for her jaw. Which she barely ate. She was 3.4kg when I adopted her. Now 3.0. It's even more noticeable since the groomer removed her undercoat and she looks very skinny.
She's had her thyroid checked and it's fine. Her kidney function was top end of normal. And other bloods were ok.,
As far as I can tell she eats and drinks well. (But shares a bowl with her sister) she's always the first to snaffle the treats! And apart from her 'horrible' heart. She's her usual bossy self.

Any one got any ideas as to how to fatten her up. (Without fattening her chunky sister)
Microchip feeders don't work, already tried that!

My vet suggested kitten food. Ollie is about 3.2kg and just swapping to kitten stuff he put on 0.2kg in 5 days
Kitten milk too. Or scrambled egg? Any high cal treats she likes?

One of my work colleagues has just suggested kitten food as well. So will try that. The problem is going to be stopping Snorg from indulging. But if I feed it as a treat on separate plates and give Snorg something else and supervise it might be ok.

She loves all treats!

Might try some scrambled egg tonight.

I've still got some high cal recovery milk. So might try making some egg with that.

Ollie is enjoying the natures menu kitten food and the applaws mousse at the minute

What a face! Assuming she's wormed regularly, so if her sister is chunky it sounds like someone is snaffling the lion's share. It sounds like you need to feed them separately and supervise, then maybe sneak Fatty a couple of snacks a day. She's lovely, OP.

Her sister has always been bigger than Fatty is. Neither are particularly greedy as they are both grazers. Fatty is always first in line for the treats. She follows me into the kitchen and gives me big eyes!

Yes, they are worked regularly.

I hope you are paying her the minimum wage in treats and not overworking her Toddler?!

Ha ha, I'm the one that's over worked, in order to keep them in the luxury they have become accustomed to!

Oily fish like tinned sardines?
(in water not oil/tomato sauce with no added salt).
(As an additional treat not as a replacement for cat food as don't contain all needed vitamins)