Well she has a point.
Your boy is 2 year old now... but 3 years old is a whole other realm as far as their behavior.
I know, speaking for friends of mine... that an older child and a baby/infant do not mix... in the same room.
But for some kids, it can work.
But, there is the whole 'safety' thing... of combining and older child/toddler with a baby or infant.
Because... a child that is under 5 years old, does not exactly have full 'impulse control.' They cannot be a babysitter and all quiet and motionless in their bedroom... and they will wake the baby. That is a given... and they should not be expected to act 12 years old.... because they are only a toddler and they are active. And then, a baby will wake too and cry... and wake the older child. So then the kids may not be getting adequate sleep etc., and if that happens... inevitably, it is the older child that is 'blamed' for that and scolded for any tweaks in sleep habits. Unfairly.
A 2 year old... you cannot expect that if you talk with him or tell him 'rules' that they will do it or understand, or not touch baby. He's only 2. Same for a 3 year old.
I used to share a room with my sister. HATED it. And, apparently when I was a baby too. My sister 'hated' me in there with her... and was not nice.
But I know lots of kids share rooms... of varying ages. And they are fine.
Its really up to you.
My own kids, well they are still young and love being near each other when sleeping sometimes, so its fine. But they are not a 'baby' and then an older child. They are already older.
I think, a baby and an older child do not mix. Frankly... even for us adults... having an infant/baby in our own rooms, keeps US awake, right? And we long to get them out of our room. So... then why do we move a baby... into the room of our other child? If a grown adult can't handle all the wakings... and sleep phases of a baby, then a young toddler certainly would not be any better at it.
all the best,
Susan