I can't speak specifically about your niece, but I can tell you about my experiences with perceptions of "normal."
I did not speak, st all, until I was 3. I grew up to be a very verbal A student. My kids used very little baby talk. Even my first never said "dada." She went straight to "dad," but she probably did it later than kids who start with baby talk. My kids are between 23 and 11 now, and all are or were honor students, in advanced classes.
Textbook "average" for learning to walk is 15 months, but VERY few people are OK with a child who isn't walking sooner. My kids all started walking between 15 and 18 months, and my inlaws were frantic. They were convinced that the kids (I have 4) were learning disabled, or that I was going to cause spinal issues like scoliosis. They told me over and over that if my children didn't sart walking sooner, that they'd have deformed spines and never be able to stand up straight. They were horrified when I told them, repeatedly, that neither the doctor nor I were concerned. Then they were amazed, each time, when my children had lovely posture and no mobility issues once they started walking. (They grew up to take gymnastics, dance, tennis, karate and fencing lessons, played Little League - all the normal kid stuff.)
Even if your niece is at the lower end of "normal," or slightly below, that's OK. "Normal" or "average" are mathematical concepts indicating where the center of a group is; some fall above, some fall below, and "average" itself covers a lot of ground.
Yes, you run the risk of upsetting them if you bring it up. If you truly feel that you must, make sure you are respectful and realize that the final decisions belong to the parents. If you do bring it up, start by asking - ask how often she has checkups, what the doctor says, what her behavior is like. If there are issues, eventually, they'll be dealt with. Being an aunt is different than being a parent - just love them all and be supportive.