I had another couple of thoughts to add...

When DS stopped taking naps, he wasn't getting enough sleep in a 24 hour period, so we shifted his bedtime 1 hour earlier and it made the world of difference to his whinging, clinginess, and tantruming. It was a good move especially now that daylight savings is here and he's up earlier and ealier each day. I really notice that old behaviour coming back if he's stayed up past his bed time or gotten up significantly earlier than usual.. he's just a horrible grump without enough sleep. So I'd highly recommend getting her to bed earlier if you can.

Another thought I had was about the negative behaviour and how that tends to increase when a sibling comes along. I read somewhere that when this happens, try to be extra tolerant of the negative attention seeking and ignore it as best you can, and find every opportunity you can to encourage positive attention seeking (which you've had some good suggestions for already). I think it's good to oblige whenever they ask for your positive attention (eg can we play playdoh mum?) because its when they're short of this that they'll resort to the negative attention seeking because they know it works. I avoid reacting to negative behaviour so he learns that it's not effective. Seems to work for me... DH who is a lot more reactive to negative behaviour always gets more of it from him than I do.

When DS became a big brother I noticed he would start asking for something as soon as I started feeding the baby (eg to sit with me, to use the computer that sits next to the chair I feed on, to do a puzzle, etc). I think he was testing if I would still respond to him. Sometimes he would ask, and when I said 'ok', he would sit with us/play computer/do puzzle for about 2 mins and then get up and wander off, happy that I was still prepared to meet his needs & wants.

I don't know if this way of dealing with it is recommended or not, but I wanted to show him that I was still responsive to his needs and avoid him feeling like the baby was dominating my time and attention. It seemed to work for us because he stopped doing it after a couple of weeks.