Sleep training 2 year old again

I need ideas. I have a 2 month old baby currently and big sis is 2 years old. The week before baby arrived big sis was hospitalized with rota virus, during that time she ended up self weaning off her milk/formula. This caused a big problem because she was used to having it just before bed so it was “soothing” to her. At bed time I would give her her bottle say goodnight and close the door, she would drink her full bottle and fall asleep, I’d go in about 30 minutes later to collect the bottle which she put aside. Now ever since she stopped the bottle she’s having trouble falling asleep. We tried giving her different milk, tried tea, tried cocoa tried juice she basically doesn’t want to take bottle or soppy cup anymore. She’s never been the kind to be attached to anything so she doesn’t have a security toy or blanket. I even tried white noise again but it didn’t work. What works is rocking her in the babys rocker (a 2day old habit) or singing to her whilst tapping her butt like a baby.... the two things aren’t sustainable. How do I get my 2 year old back to sleeping alone in her cot in her room.

I mention the 2 month old because generally big sister is very whiny and not yet used to having a sister so that in its self makes her more cranky and more so when she’s sleepy.