AT this point, this is a pure discipline issue. She should not be able to get out of her room and into her brother's room. Not sure what you mean about creams - is she getting into the bathroom cosmetics or into her mom's personal stuff, or the diaper cream, or what? Everything should be out of reach, and the door should be locked from the outside, if nothing else works. She's getting plenty of attention - even if it's negative attention - by doing what she's doing. She needs a specific routine that is followed exactly every night, with no caving in by parents - granted, that is so hard to do. You can't force her to sleep but you can confine her to her room and have repercussions/consequences for getting out of bed and getting into stuff. They must take away privileges, or clear her room of temptations, etc. You'd be surprised how effective it can be to take the stuff she gets into, put it into a box, and store it in the attic, only to be brought out when she follows the rules. Be sure you aren't just storing things on high shelves - she could decide to climb the bookshelf and pull it over on herself. Either that or the bookshelf should be anchored to the wall - something the child safety experts recommend anyway.
I would let her keep comfort objects like a blankie or special stuffed animal, a binkie if she still uses one at night. We also let my son keep books. But everything else went into storage and only came out when he earned it. Yes I know it is a hassle to put things away every night but this current routine is also a huge hassle. And no one is getting any rest, which isn't good for them.
She's doing this because she is getting a reaction. If they can force themselves to just matter-of-factly explain, on her level, that these things will go away if she doesn't stay in bed, and then follow through (because she probably won't stay in bed!), then it will work in a couple of weeks minimum.
Certainly anything she can use to calm down and block out stimuli is a good idea - like a CD player, a darkened room, a white noise machine - but only if they help rather than become something she messes with.
They can also implement a sticker chart or some other incentive program - although getting her stuff back may be a big incentive. They have to make it simple: it's night time and time for rest. You are not allowed to play after bedtime. We have a routine of (for example) bathtime, book reading, 1 lullaby, and light's out. Follow that routine every night. They probably had one but she either grew out of it or her physical abilities developed enough to allow her to climb out of the crib. They just have to put in a new routine and stick to it. It will probably be awful for the first few nights - lots of crying and fussing - BUT they will be doing her a favor by teaching her to sleep, and they will certainly be doing themselves and the little brother a favor by having calm time and no unnecessary clean up of creams or whatever. Kind of like doing the right thing by putting her in a carseat whether she likes it or not, by not letting her run in the street whether she likes it or not, and so on. It just can't be negotiable. They can't make her sleep, but they CAN prevent her from doing anything else that's fun.
Good luck - it's not easy. If they can take the frustration and the constant going-in-her-room out of the picture, it will ease up. They just need to pick a strategy, agree on it, and stick to it.