I agree with LeQueen - routine and consistency is the key!
We had a nightmare first few weeks with DD where she was really collicky in the evenings and would not settle. We were keeping her downstairs with us.
At five weeks, we decided we needed to reclaim the evenings.
We started bath, bottle, bedding and were amazed that on the first night she slept 8 - 3:30am. We tried dream feeding a couple of times, but it didn't stop the 3am waking so we decided to just go with it.
Gradually the 3am waking got later and later and by 12 weeks she was going all the way through.
We had blips - about 4 nights each month (coinciding with growth spurts) she would wake up and want a bottle. Because we identified the growth spurt, we felt it better to feed her. Usually after the third or fourth night she would be going through again, however, she sometimes got into the 'habit' of waking up, so on the fifth night there would be no milk and we'd stay with her until she settled again.
She still has the occassional short periods of days when she will wake in the night, but they are few and far between now and still equate to illness or growth spurts.
I saw that you said you have a routine but that the bed time can vary up to 8:30. Does the 2.4 year old have a nap in the day still? If not and she is getting up for breakfast at 7:30, she could very well be overtired - 2 - 3 year olds generally need about 13 hours sleep a day. Even if the younger child is sleeping in the day, she could still be not getting enough (at that age, my DD slept about 16 hours a day including naps - soemtimes more).
Overtiredness will lead to poor sleep patterns. I would bring the bed time forward with both children being in bed by 7pm, after bath and drink, with lights out being no later than 7:15pm. Stick with this rigidley until they are both going to sleep quickly and sleeping through - you can then become a little more relaxed if needs be.
Finally - does the 9month old sleep in blankets or a sleeping bag? If you are not using a sleeping bag, it might be worth giving it a go, my DD slept much better in hers - it was also easier to settle her if she did need to be picked up.
HTH