Doesn't every woman want to fall in love, get engaged, marry the person they love and possibly start a family? These milestones aren't exclusive to you. I could very well say that your SIL is trying to copy me!
I don't think 19 is too young to be thinking about these things. At 19 I met my beautiful fiancée and we became engaged almost immediately. Our wedding is planned for 4 months and we have a beautiful daughter.
After our DD was born my best friend said she wanted to have a baby. I didn't think she was copying us. She's allowed to have children when she wants. I wouldn't dare look at someone's circumstances and judge whether they're 'suitable' to be thinking about TTC or getting engaged or having a wedding.