How contagious is strep throat!??? My newborn was just around it

Omg im freaking out my mom kept asking for me to come over so she can see my newborn she’s 3 weeks old my mom only met her once so I took her over there and found out my nephew has a strep throat !! He lives with my mom and also sleeps in the same bed as my mom does that mean I literally just exposed my newborn to that??? She’s not sick but my nephew is and he’s always around my mom literally every second and then my mom said “he’s on antibiotics he’s not contagious anymore” Idk wtf to believe