I agree with MyName.
If it's only for a couple of years, I'd let eldest keep her own room - she might be hitting puberty sooner than you think, emotions all over the place , needing her own space. With all that to contend with and a new baby, she'll surely need more, and certainly not less space. And at that age, she'll need an increasing amount of privacy - she can't have that sharing a room with a toddler brother.
Also, the little one will need to go to bed earlier than your DD, so where would that leave her at his bedtime? Tip-toeing around him in what is her room?
I think it should be you, not your DD who takes the hit in terms of having to give up their bedroom.
We did have a DD, then a 9 year gap, and then a DS. There is no way I'd have asked her to share her bedroom with her brother. I know your age gap is lightly less, but when you've had your own bedroom for 9 years, you aren't going to give it up without a significant amount of resentment, I wouldn't have thought.
If it were me, I wouldn't do it unless me and DH were prepared to give up our bedroom. (There's no way my DH would agree to that, so for me it would be a "No", but it might work for you.)
We were in the same situation, and we didn't have another child until we had more space. Asking DD to share was out of the question, and our third bedroom (it was more like a big cupboard with a window) literally fitted a cot in, so we had to wait.