Hmmm...baby can stay in your room until she or he can sleep through the night, hopefully cutting down the disruption of sleep for the older kids. But after 6-12 months, your baby will need a space that's geared towards a child his or her age. Whatever you do, involve the kids in the decision. They might come up with an idea that you didn't think of.
My first thought is, is there a basement that you can finish partly? Or a porch you can enclose, insulate and heat? If you don't, and you have decent-sized master bedroom, would it be possible to partition that room into two rooms, a bigger one for the boys and a smaller space for the girl? My ex has a two-bedroom condo and the kids stay in the master bedroom and he has the smaller room because when my step-daughter (who is 20) is home from school, there are three kids who need sleep space. My younger boys are 12 and 14 and only spend one night there a week at most. When she's home, they have one side of the room and she has the other. He's in a rental so he didn't put up a wall but if he owned the place, he would have actually added a wall to split that into two separate rooms with separate doors.
If at all possible, I would try to find a way for the 14-year-old to still have her own space when she's there. As a teenager, her schedule will be very different from that of a baby and there will be all kinds of things that she'll have (makeup, jewelry, electronics, etc.) that you would need to keep out of reach of the baby as the baby gets mobile. You don't want her coming in from a night out or work when she's 16 and waking up a toddler at midnight. You also don't want her to feel like she's getting pushed out for dad's second family, living out of a suitcase and sleeping on a pull-out couch. I guess it depends on the relationship with your step-kids too...if things are good and they (especially the girl) will welcome a new sibling then it may not be a big deal for her to have to share a space with the baby when she's there. If things are difficult though, or if it will prompt her to not come on the weekends, try to make space if you can.