Based on the definitions posted by MaryHenley this is about semantics.
Two people sharing living quarters and participating in sexual relations are cohabitating. This is fornication and a grave matter.
Two people sharing living quarters and NOT participating in sexual relations are “sharing living quarters.” This is not fornication and not a grave matter.
With regards to two siblings living together (same or opposite sex), without evidence of something inappropriate, to assert it is scandalous requires one to make a rash judgment which is a grave matter. See below from the catechism.
While in college, two of my female cousins (cousins themselves) shared a efficiency apartment their first year of graduate school and shared a bed. I’m wholly confident that nothing inappropriate ever occurred there. As a family, we enjoyed the stories of their “battles” as we think they were less tolerant of individual foibles and habits they might have been if they hadn’t been related.
Furthermore, my son, another female cousin (I have 21 cousins), and two of her friends shared an apartment for a summer while in college. Again, I’m wholly confident that nothing occurred inappropriately between the roommates. In fact, I liked the idea that my cousin was w/ my son as it discouraged any inappropriate behaviour on his part (at least in that apartment).
Finally, I was once stationed in another town for work (I was married w/ children and about 35) and living in a motel for three months. While there, another female cousin (yes I have alot), had the opportunity to have a one month internship in the same town. For that month, she (about 21 years old) and I shared a single motel room w/ two double beds. We knew it didn’t look good so we tried to be discrete (never going in the room together). At the same time, if she hadn’t gotten a free room, she could never have taken the unpaid internship which she desperately wanted to do. As a side benefit, it allowed me to really get to know a cousin who by age and distance I never really knew that well. I now literally consider her a sister. The only time when things were stressful was in the morning during the battle for the bathroom. Many evenings we both got back around 8 p.m. as we worked late and ate pizza in the room. And frankly, I look back on it as a lot of fun living like a college student!
IMO, there is way too much energy wasted on what appears to be scandal without facts and too little on actual known bad behaviour. If parents and friends would be vocal about that which they know is wrong and not being critical for “appearance”, I think there would be more accountability and better behaviour.
From the Catechism:
2477 Respect for the reputation of persons forbids every attitude and word likely to cause them unjust injury. He becomes guilty:
*]of rash judgment who, even tacitly, assumes as true, without sufficient foundation, the moral fault of a neighbor;
*]of detraction who, without objectively valid reason, discloses another’s faults and failings to persons who did not know them;
*]of calumny who, by remarks contrary to the truth, harms the reputation of others and gives occasion for false judgments concerning them.
2478 To avoid rash judgment, everyone should be careful to interpret insofar as possible his neighbor’s thoughts, words, and deeds in a favorable way: