Sounds like she's drinking 2 cups right before bed, which is a lot, but I can't tell from your request how much she drinks in total during the day. With my son, we would give him milk in the morning when he woke up. He'd have some water with/before lunch. He'd then get a cup of milk when he woke up from his afternoon nap (between 3 and 4:30 pm). I think then we didn't give him too much to drink with dinner at all, and then he'd have a cup of milk before bed... He, too, sometimes soaked the sheets. At night, we started to use Huggies Overnights - what a life-saver (sheet-saver)!!
But the other thing is, if you can, I would spread out the liquid-drinking so that it's not mostly right before bed. Try giving the cup of milk that you normally give with dinner after nap-time instead. That way it's earlier, AND she's not filling herself up with milk during dinner either (making more room for veggies, etc!)... Then just give her the milk after dinner/before bed. ...and use those Huggies Overnights!
Of course, this all changes when you potty-train her! My son is now 3 1/2 and potty-trained, but in order to keep him dry at night, we have to cut off most liquid drinking after his afternoon cup of milk. At dinner or after dinner, he only gets 2-3 oz of water. It seems to work well, but we just have to make sure we offer water/milk more often in the earlier parts of the day to make up for it.
Anyway, others have probably given you similar advice, so I'm sorry if this is a repeat! Good luck.