oh bless her.
I would say that 2.6 is very young for depression. yes, of course, she will still be finding adjusting to the new baby very difficult. It is unsusual though, at her age, for her not to be pretty vocal about it, and more obvious, if this were the reason. She's note yet developed the emotional equipment to keep inside for long, or the sensitivity either.
I would have thought there was a more physical reason for her being withdrawn. I'd really take her to the GP for a good check up. possible some bloods, and a examination of her ears, throat, tummy etc... If she is drinking less than half of what she normally does, you need to keep an eye on her hydration. Lack of food ad drink can make a little one feel pretty lousy, sleepy and withdrawn.
I had experience of this myself recently when my DD (18months) was i hospital being treated for meningitis. It was horrible. the most upsetting.worrying bit was her being so withdrawn and floppy, it was amazing how much she perked up on IV fluids.
I'm not suggestig she needs this, of course, but maybe just try encouraging more fluids into her. I was advised to give her any food she might eat, all the 'crap' you would normally not give too often, as well as offering a balaced meal. Also, i bought loads of smoothies, fruit juices, fruitj mineral waters for children, everything! it took us a week or so, as she was quite poorly, but still, now I'm keeping up with it all as she lost alot of weight.
Hope some of that helps