My daughter is also 17 months and I had/have the same concerns. At her 12 month checkup the form the dr gave me said she should be saying 2-8 words other than mama and dada between 12 and 15 months. Well at 12 months Madison said "hop" and "mama". That was it, and it was a rare day to hear either of them.
At 15 months she said the same words, nothing was added, so I told the dr at her 15 month checkup. Different dr this time and she told me not to worry about it yet, give her at least till 18 months, that she (the dr) didn't speak till she was 3yo. She said if I really, really wanted to she'd give me the # for a speech therapist for an evaluation. I agreed to wait.
Around 16 months she said 'up', then 'dada', then bubbles. Now at 17.5 months she says Mommy (no more mama!), Dada/Daddy (depends on her mood), up, hop, bubbles, Nana, bye bye, uh oh, hi, and pop. I know it's a weird collection of words, but hop and pop come from her favorite books. She just started saying hi 2 weeks ago and pop for the 1st time yesterday. So total she says 10 words. My mom claims she says 'Bailey' (her dog) but I don't really count it cause it sounds nothing like it. I know what she means cause it's the same sound everytime she's referring to the dog, but not a real 'word'.
We'll still be talking to her dr on the 31st though cause 10 words at 18 months sounds pretty low, but I'm happy that she's shown a lot of progress in the past 3 months.
Oh, and Madison is our only child, since you mentioned something about kids with older siblings.