How can I make a thesis statement using number of siblings (as a dependent variable, I guess it is a continuous variable, right?) and the use of social media (number of social media accounts as an independent variable) and number times participating in social events (independent variable)? I need help wording a thesis correctly, since I have the data to measure all of those variables. What other variables would be useful to analyze in correlation with family size? Maybe the level of education people are expected to reach?

The independent variable is the variable that you, the experimenter, are changing in your experiment. Therefore, the independent variable would be number of siblings. This is a discrete variable, not a continuous variable. Discrete data can only have certain values, while continuous data can have any value in a range. Since number of siblings is measured in whole numbers (O, 1, 2, 3, 4, etc.), it is a discrete variable (height would be a variable that is continuous since it can have any value in a range).
The dependent variable is what you measure and what you think might be affected by the change in the independent variable. In this case, your dependent variables are the use of social media (number of social media accounts) and number of times participating in social events. The level of education that a person expects to reach or has reached would be another dependent variable that you could measure that might be related to family size. The thesis statement could be that you are investigating the effect of family size (independent variable) on use of social media, participation in social events, and level of education attained (all dependent variables).