When is it safe for family to kiss your child?

My boyfriend comes from a very big family. But mostly him, his mom & his brothers families are all very close. His kisses his 3 year old niece on the lips. I don’t come any family lol but I’m 38 weeks pregnant with a boy & I know you’re not supposed to let anyone kiss a newborn bc they can get sick. Well at what point does that become safe? Obviously I’m not going to let outside family randomly kiss my child on the lips but more so the closer family. He doesn’t see a problem with it when baby’s a bit older (like out of the newborn stage IM GUESSING) I’m a first time mom & I don’t really know what’s right or wrong or what’s okay etc. Advice, is it normal or not or when? I was never around baby’s or young humans