Nursery or no?

Im 25 and 14 weeks pregnant. My sister is a 21 single college student. She is overall an intelligent girl but is already concerned about MY parenting and very upset with a lot of stuff. My husband and I are renting a big 3 bedroom house. We have two rooms rented out and we live in the master bedroom with walk in closet and bathroom. My sister is so upset that I am pregnant with roommates. She said they should moved ASAP and I should get the nursery started. First I did not plan on a nursery because I feel it's best the baby sleeps in our room in his basinet. I plan on arranging a section for him in our bedroom. Then later on once he is older or I have another baby I will dismiss the roommates and let them have their own room, or we may even purchase a house by then and not need roommates. I personally don't think a nursery is necessary but my sister got extremely upset about this idea and turned super red saying how important a nursery is and that the baby needs to learn to sleep in his own room alone. I don't understand why is this such a big a deal......