“I ran into someone I haven't seen in years.”

Earlier in the day, you saw someone that you used to know but haven't seen for several years & were surprised. You want to tell your sister about it & say:

I ran into someone I haven't seen in years.

run into (someone)

This means to meet someone unexpectedly.

haven't seen (someone) in years

You can use this phrase when talking to anyone - it's not especially casual or formal. You use "in years" when you can't remember how long it's been since you saw them, but you know it was a long time.