Emsiewill,
when I swapped ds from a cot to a bed, we put a cabin hook on the outside of his door. the door is open about an inch, but won't open any further. So we can hear him if he cries, but he can't get out. We have only used it on nights when he has got up, we put him back to bed and hook the door shut. then he cries for a few minutes, but eventually gives up and gets back into bed and sleeps. (controlled crying)
I suggest you try this. It seems like your dd2 enjoys the social interaction she gets from coming in to you or her sister. If she can't get out in the first place, you cut that out. I think you really have to be strict, otherwise it really is just an entertaining game for her.
on the first night, I would fasten the cabin hook, and tell her that she can't get out of her room in the middle of the night. When she wakes and tries to get out in the night, tell her that she cannot leave her room, it's nighttime and she must go to sleep (don't go in to her) then go back to bed yourself and let her cry it out. obviously if she starts to get really upset you will go to her, but be as strict as you can. Once she realises she can't win, she should settle thru.
While ever you get up to put her back to bed, she is getting the attention she wants.
Don't forget that the first time she let's you sleep thru till morning she needs tons of cuddles and maybe even a special treat.
Obviously you may need to go back to nappies or put a potty in her room (risky re spillage) until the issue is fixed, but worth it I think.
You really need to get this fixed before she starts a full school routine, otherwise it will affect her school days.
HTH
love