Why Won’t Our Little Girl Sleep?

M’laddo had silent acid reflux when he was really little and until it was diagnosed, he was often up every 15-20 minutes during the night for a sip of milk to ease his burning throat. As he got older and the reflux was treated, he began behaving like a normal baby, dropping the bottles during the night as and when he didn’t need them any more. By 10 months he was sleeping through the night, even if his “night” sometimes ended at half past 5 in the morning.

The wee lass has never really been like that. Sure, there’s been the isolated night where she’s almost slept through, or just had a bottle at 10pm, or 2am but those have mostly been prefaced by a night of no sleep whatsoever or a dose of Tixilix. She’s one in two weeks time but most nights wakes up as much as she did when she was a new born.
I don’t want to sound like I’m comparing our kids and coming out with the conclusion that one is better than the other, because that’s not what I’m doing at all. The wee lass is a lovely sweet natured little thing, with a ready smile and a lovely cuddle, when she’s not screaming through exhaustion, much like M’laddo is energetic and full of joy when he’s not being a little sod.
We’ve tried everything- feeding her up in the evening so she’s full, giving her a lighter evening meal to make sure she doesn’t get a tummy ache, making her room cooler, making her room warmer, having it in total darkness, having a night light on, and so on. We’ve tried ignoring the crying (in case we were rewarding her with milk etc for making a fuss)- all that happened was she woke her brother up and we had two little ones to settle.
Right now at 7.30 she’s trying to climb onto the sofa. She’s been up for 3 hours, and had 3 bottles during the night, plus 4 or 5 settling downs prior to 11pm. By mid afternoon, even if we can manage to get her to take a nap, she will be so tired that she will be grinding her face on the carpet and making a terrible fuss. She has a good bedtime routine, bath, bottle, bed at 6.30pm. We daren’t put her down earlier as we have the sneaking suspicion she’d be up at 3am, and she wouldn’t have her bottle if she went down any later.
Short of drawing pentagrams on the floor, there’s little else we can think of to try- mainly because she wakes her brother up if we leave her- so has anyone got any suggestions?