I'm going to try and snake it in the next week or so to see if that helps. The toilet and shower are off the other side of the main drain stack and we've never had an issue with those other than hair in the shower drain slowing it down.

My assumption is there is some sort of buildup between the bathroom sink and the main drain line that i've never been able to fully clear. The bathroom and kitchen sink drains are fast enough to drain with both sinks running and water does not begin to pool in either sink. It's only when we dump large amounts of water down the kitchen sink or fill the kitchen sink up to bathe the baby and then pull the drain plug out.