i was 19 when i got pg (i'll be 22 in june) so not much older then your niece. so i have some idea of what she might be feeling right now. you seem to really want to help your niece so i am going to be honest with you. please don't take offense to what i say i am just trying to help. i think it is awesome that you want be a source of information and support for her, she will need that! i also believe in the importance of breast feeding, i think it is even more important with unexpected pregnancies.
when i read your post the thing that stands out the most to me is the negativity behind it. you do not seem to have much faith in your niece or her boyfriend. you explain her decision not to breast feed by saying she is immature, the pregnancy was unexpected, and your niece is making decisions about what is convenient for her not what is best for the baby. this is basically what most people think in response to young parents. but try to think of it like this. would you value the opinion of someone who thought you were immature and selfish?
here are my two biggest points. first and most important being young and unexpectedly pregnant does not make her any less of a mother so talk to her like you would if she were 30 and married. always work under the assumption that she is going to be an amazing mother.
the second is that because it is an unexpected pregnancy she didn't have the months of planning most people have to think about how she will mother her child.. she will need a few months to catch up. give her information but she will probably be more ready to talk once baby starts moving. if she is already at that point then just keep talking to her as if she were your best friend and you are sharing your experience. my family endlessly supported me and told me what a wonderful mother i would be (and am now) they absolutely respected my decisions and because of that i respect their opinions and ask their advice.
oo and the third.. if she expects her b/f to stay with her just go with that.. she knows there is a chance he wont.. but thats not something people need to keep reminding her. DP and i had been dating 5 months when i got pg, DS is 16 months now and we are still together.. so it is possible.
so heres how i would (and do) present bfing. remember who your target audience is. don't talk like your teaching an infant development class. everyone knows breast is best your not going to win her over with nutritional information. you have got to make it sound Cool!
try and sell the whole AP package (it makes more sense that way)
co sleeping and nursing is much easier then FF roll over latch on back to sleep
FF is to much money and work who wants to spend all day washing bottles?
wraps and slings are sooo much cuter then those bulky strollers!
you could just pop baby in the sling and stick a diaper in your purse to go out. she can nurse in the sling no problem! no bulky diaper bag and PITA bottles.
don't forget the weight loss!
i think girlmom.com has some stuff on how great breastfeeding is. there was also a post while ago about a pro breastfeeding website with young moms. oo and show her all the hilarious nursing themed onesies (i like mother sucker)
btw i am being totally serious remember your target audience. take her shopping for nursing bras and nursing shirts, play up the convenient thing... etc. she needs friends right now, be that friend, be supportive, treat her as an equal... the closer you are the more likely she will be to listen
if you think she needs to hear it from someone closer to her peer group PM me. i can make breast feeding sound like the coolest thing on the planet.
if it makes you feel any better my son is intact even though we were young and he was unplanned.. dp is circd but when i said there was no medical reason he said we wouldn't do it

he is also against infant ear piercing. teens are big on personal choice.. tell her it should be his decision not hers.