http://www.cdc.gov/ncbddd/autism/actearly/

If I were in your position, I would be hesitant to say a whole lot to my sister, too. I think I might just ask questions, and phrase them in an interested and general way. “How many words do kids this age normally have in their vocabulary?” and “This puzzle says it’s for kids age 3-5. Doesn’t she like puzzles?” If you phrase it like she’s the expert and you’re just wondering, she might be more receptive. If she expresses any concern, you could say “Well I don’t really know anything about that. Maybe you should check in with her doctor and see.”

Approaching your sister will be a different problem… some people are not receptive to the idea that there may be something wrong with their child. I would mention my concern, state what I had learned from research, perhaps find reference to what she should be able to do at this point (check out HELP Strands, or the Denver, developmental profiles to get an idea of what she should be doing at this point), and suggest that she have her child evaluated (for free) from the school district.