Hi, most "advice" for pregnant moms regarding health and pregnancy is good for any age. That being said, more than advice your cousin probably just needs someone to be with her and support her. Maybe that means having her join an app like this one so you can learn together and celebrate milestones, maybe it means being someone that she can talk to with any thoughts or concerns and rather than have you come up with an answer just being there and journeying through it with her. If you've been pregnant before you can definitely share your experiences, but more often than not we would rather have a friend and a listener than someone who is giving us advice (unless it's specifically asked for).
Maybe download the bump app together? There are a lot of good (and not alarming like Google) articles about preparing and journeying through pregnancy.
I'm not sure how close you are to your cousin, but I agree with a lot of what @linh18 said. I think it would help to just let her know that you care about her and are there for her to support her, that you are there for her if she wants someone to talk to or if she has any questions. And then see how she responds. Perhaps check in with her when you can, but allow her to decide what she's comfortable with sharing. This is probably good advice for talking to anyone who is pregnant, but I'm assuming it might be harder to navigate through unsolicited advice/questions at that age.
Re: 16 and pregnant..
Maybe download the bump app together? There are a lot of good (and not alarming like Google) articles about preparing and journeying through pregnancy.