I recommend you to produce a survey, with a set of questions where if the score is lower than let say 5, then that person is introvert and over 5 the other is extrovert.
You will need to have real biological siblings living/growing up together to answer the survey. Each sibling answering a copy of your survey independently in their area (no contact during the survey).
Just go around your neighbourhood and get your classmate and your classmate's siblings to answer them. About a dozen of validated (e.g. answer all the questions and correctly) returned answers will be good for your purpose.
Split into 3 groups (first born child vs middle child vs last born child) and do one-way ANOVA.
You analyse the result from the survey instead of asking people to randomly give you results. Asking for people to randomly give you their comment is not science.
Also, this topic is more of psychology than biology.
How about growing beans in different level of salinity?
So you grow the beans, start off in the same environment and you let them sprout. Then you water them with the same amount of water everyday but the plant receive water of different salt concentration (no salt, 1 tea spoon in 1 litre, 10 tea spoon in 1 litre). Measure the growth (or death) and make a report/poster or other form of assessment your teacher ask you to do, with regards to the environmental problem of salinity.