Immune system questions ( advice needed)

My little one is a year now and has never been sick. My baby stays at home and during the day my mom watches my baby. She is not exposed to anything and my sister said that it’s not healthy she needs to build an immune system. Where still in a pandemic so her interaction with others are very limited. Can someone shed so insight? I know eventually she will get sick…. I just don’t want to intentionally expose her just to build an immune system.