This might be outing but there’s 14 years between me and my sister and 16 years between my brother (same mum, I had a different dad..)
First few years my sister lived with her dad and would just visit. She didn’t move back in until she went to uni. My brother moved out after uni (went to a local uni) but moved back in a short while after breaking up with his then GF.
I remember the expectation was if they were both at home I’d have a ‘put you up’ in my parents room (which i loved), but most the time I’d just bunk in with my sister. Quite often I’d wake up to her in with me (guessing she’d come home unexpectedly/end up staying over).
Even now, we’re both married, but if we’re in the same house we’ll end up bunking in together. If I hear her husband leaving for work and we’ve been staying I’ll slip into her bed.
Slightly different view, but I made sure to have three kids of a similar age. I hated when they both moved out for good and got their own lives. The house always seemed quiet and I found it quite lonely.
The only two options are for your DD to be in your room when the 18 year old is staying, or for your DD to share with her older sister.
Renting a 4 bed is financial suicide and it’s totally inappropriate for your DD to share with her teenage brother.