New Baby/Guest Room

My son’s bedroom is Jack and Jill with the guest bedroom, which is across the house from the master. He’s currently 9 1/2 months old and will be 13 months when the next baby arrives. My husband wants to keep the guest bedroom a guest bedroom (his mom and her boyfriend stay with us occasionally) and have the two kiddos share a room, but I think it’s weird considering we have the extra bedroom.

I’m also debating about putting her in there from day one. I’m only worried since the bedrooms are Jack and Jill, she’ll wake up her brother.

We had our son in our bedroom for 6 months after he was born because his nursery was upstairs and the master was downstairs. We moved to a one floor house so that’s why I’m considering putting her in her own bedroom from the start.

We have a four bedroom house, but my brother in law lives with us so he occupies the other bedroom.

Has anyone been in this situation before? Or does anyone have any recommendations/suggestions? Thanks!