Need advice on what to say to my 11-year-old-niece?

Although your intentions are good, you may be doing more harm than good by talking with her. Not to mention, this is her mother's job, not yours. If it is really that important to you, I would suggest speaking with her mother and letting her know how much can be seen when wearing the looser clothing. Offer to take her shopping for cami's to wear under those clothes if she still doesn't want to wear a bra. If you tell an already self conscience 11yr old girl that you can see her breasts, she will become mortified. If her mom doesn't do anything after you speak with her, then just buy her some cami's as a gift and then leave it alone.