It could well be your new bitch's 'den' instinct coming into play - any expectant (as she feels) bitch will defend what she sees as her future birthing place and home for her pups.
It may also be a bif of assertion in pack terms - even though she is the newcomer, there's nothing to stop her advising of her own spaces in the house - your other dog will wither accept her dominance, as he seems to, or they will spat and snarl until an order they can both live with is established. This seems to have already been sorted out between them, but if your other dog decides to make a play for the 'top dog' position, your new bitch will resist, and there will be tension. If this happens, leave them to sort it out for themselves, unless it gets prolonged and nasty, but that is very rare - the 'top dog' makes his / her position clear, and everyone else falls into line.