L.C.
Is the little one still in a crib?
We had this problem and even up in the middle of the night ready to play. I'm not saying this is going work for everyone, but we invested in bed tents. For one thing, they loved staying in them because it was like their own little cubbie or cave. For another, it was kind of out of sight out of mind. Could be your younger one woke a little, looked around in her own room and nothing was going on so she went back to sleep for a bit (what ours did), but now with her sis in the room she sees her favorite playmate and is ready to rock and roll. Maybe if she couldn't actually see her inside the tent, she would be more willing to go back to sleep.