Advice on almost 4yo bedtimes

[YukoandHiro]

Dd is 4 in august. For about 18 months she has been straightforward at bedtime - always down by 7.30. For a long while I had to lie with her but in the last six months she's been better at settling herself after stories and songs.
Suddenly out of nowhere it's a nightmare. She's up and down for hours, won't settle, til she finally crashes out - often after 9pm. I'm up with a baby a couple of times in the night too and this is wiping me out (my husband works evenings).
The baby is 8 months old not a newborn, so I don't think it's a reaction to that. It started well after she'd settled into being a big sister.
Any thoughts on new bedtime techniques that might help?

What have you tried so far?

Is this recent as in the last month or so? If it is then it's probably just the lighter evenings and will correct itself when the nights are darker.

Black out curtains and a couple of conversations about how it isn't always dark at "sleep time"(we've had to stop calling it nighttime to avoid confusion)

I think they just have waves of change, there doesn't have to be a deep issue, just a bit unsettled at the moment.
We just ride with it, our sone is 4 at the start of August, he needs me a bit through the night at the moment due to his cough/chest.

But he goes to bed at 17.30 asleep by 18.00 awake at 06.00 normally.
At the moment he's awake about 05.15, but we have a grow clock and he knows he can't leave his room until the sun comes up at 06.30

@BunnyRuddington I've tried moving bath and bed later, also tried bathing both children together then letting her have quiet free play in her room while I put the baby down first (baby likes to be asleep early so that routine suits both) followed by lights out, I've tried audiobooks and sleep stories. Also tried offering weetabix as a supper. One night when I was desperate I even tried letting her watch a sleepy story on the iPad in bed (the baby was being tricky due to teething - I wouldn't normally do this).

I do think light nights are a part of it - we have curtains lined with blackout but it still creeps around the edges

Thanks @readwhatiactuallysay - just hearing you say it could just be a short phase makes me feel lighter and more relaxed!