First of all, what exactly are you referring to when you say "lags", you mean lag spikes or generally high lag?
I recommend doing a ping test between each of your computers and a large website like google or smashboards. Here's how to do a simple ping test to google in windows:
Open the
command prompt and enter:
ping google.com -n 50
The part that say "time=___ms" is the ping. Both of you should do this.
If the ping varies by more than a couple ms, your connection isn't very stable and it's your connection that causes the lag spikes. If this is the case it might be because of other people using bandwidth at the same time, a bad router/modem, a bad wireless card or simply bad wiring.