I would not, have them share a room.
The eldest, is in no way old enough to supervise her baby sibling or to be "responsible" for her baby sibling.
And the baby... is young and will wake and cry at all hours and go through all those baby phases and need, you.
And it will interrupt the elder sibling's sleep, to say the least.
Sharing a room will not guarantee that they are close "buddies."
I shared a room with my sibling... and I HATED it.
It was the most terrible thing, and I still remember it as an adult.
Sharing a room with my sibling, was just awful.
I would let each child have their own room.
They are 3 years apart.
And the eldest... will need her own privacy and her own things and her own friends.
Of which, it will not be suitable or "safe" for a baby.
The baby should be in your room.
Or in her own room.
And the eldest should be in her own, room.