Family man here. DON'T DO IT.
My wife has a nice 2015 4Runner Trail Edition. I have a 2013 FR-S. We have one kiddo. If the whole family goes somewhere, we take the 4Runner. But ~90% of the time, I'm the only person in my FR-S. The other ~5% it's me and the little one, and the last ~5%, all three of us fit into the FR-S. We make it work out just fine, if anything, we've debated getting a beater 4 door sedan to minimize mileage on our nicer cars.
If you want an STI because they are badass cars, then I say go for it. But I test drove both a 2015 WRX and a 2015 STI, and it's not the same as my beloved FR-S (which is also paid off to boot). It's a big decision, act carefully friend!