Another baby

We're thinking about trying for another baby, I have a 15 month old girl. I have major anxiety when my 15 month old is around my SOs sister's 3 month old son, because I'm so scared she'll hurt him. Will it be different with my own? Like I know I'll still be nervous and what not, I'm just wondering what the differences are between having your toddler around someone else's infant, and your own. And wondering about your own experiences