I would probably take easy road first - check she's full when she goes to sleep.
Maybe give her snack about 30 minutes before bedtime and then milk feed so she will go to bed full. See how long she can sleep for? Try for a few days and see if there are any changes. My son is almost 2 and I give him 240mls of milk before he goes to bed. Sometimes he drinks 480mls even when he's had big dinner.
If she refuses snack and/or milk feed before bed then waking is probably a habit. Habit is harder to break. You should give her water and then put her back down to sleep. She will cry loudly for the milk but if you give in she will keep crying every night until you give it to her. This will take some time and lots of crying for everyone. But I feel better to fix the problem immediately than live with little sleep for many years.
It not easy but the older they are the harder it is to change their ways, the stronger they are and the louder their crying. Also with #2 you can't be getting up at night for both children.
It's difficult to know she is crying and you're not giving them of what they want. But the other side is that no one is getting much sleep. If you preservere it will work.
Hope this helps. Let me know how it goes.