Sharing a Room

Just yesterday, the Mushroom announced that he'd like to sleep in the same room as his sister Molly so they both won't get scared.

He's currently going through some stuff.  Spiders, mice, darkness... you name it, he's scared of it.  He has a night light, mouse spray, spider spray, constant reassurance from us that there is no need to be scared of anything that is one hundredth of his size... (don't make me do Math, my head hurts) but you know what the rationale of a 4 year old is like...

Now I can count on one hand, how many nights Molly has woken up in the last month at night and I can count on one finger how many nights that Mushroom has slept in his own bed in the last 8 months.

WTF

I am opposed.  Vehemently.

Juffin thinks it's a good idea so we can turn one room into a play room and we can get all the f-ing toys out of the lounge room.

Between you and I, we know that the statement above will never happen.

So let's go through my reasons for not wanting to do this:

Mushroom has a night light
Molly is still in a cot
Mushroom still needs someone to sit with him when he goes to bed (Yes.  It's fucking ridicuolous.  Don't even get me started!)
Molly is a light sleeper
Mushroom is a nutcase and wakes multiple times at night and creeps into our bed, if we take him back he becomes hysterical and WILL wake another child who would be SOUNDLY SLEEPING in the same room
Molly is a fantastic sleeper and I literally cannot cope with one child in my bed let alone two.  I'm too fat.

Now I don't care that they're opposite sex, they're kids.  I think it's sweet that he wants to share a room with her.  I just don't think that my girl will enjoy having a room-mate.  She's a very independent little thing and I think she likes her own space. She is still breastfed before bed and literally says, nigh night Mummy when we put her down and she rolls over and goes to sleep.

Oh if the first born would do that!  We'd be in HEAVEN!

I guess we could trial it and if it's shit, go back, but will that stuff the good sleeper up forever?  I like my three hours of unbroken sleep!  I need it!!

What's your advice fellow parents?  What would you do?

Their hair is almost exactly the same colour!