My puppy is howling even when someone’s home

Hi, So recently our family lost our beloved dog when he was only 2 years old. Since my little sister was really feeling down and felt alone we decided to take a puppy from an animal shelter. Hes only 3 months old and he’s a really affectionate and a sweet little guy. But the thing is that in the 4 days that we have had him he got crazy attached to my sister: even when she goes to the bathroom at night he starts stressing. Now she’s been gone to school the second day and I’m home but he’s still going crazy when she’s not here. He’s howling, barking, and scratching every corner of the room. Then he lays for an hour quietly and starts again. I don’t really know what to do, when she’s at home he feels good and relaxed, but whenever she’s not there, doesn’t matter if someone else is at home he’s very dramatically howling and going crazy at some moments. I didn’t experience this with my last puppy, so I don’t really know what to do. Is it going to pass? Please if someone else experienced this, help❤️