So an issue with the sewage ejector pump. I've got a downstairs bathroom that flows into a sewage basin. Every time the pump kicks on, i get water forced out of one of my kitchen sinks upstairs. I've got studor vents on both kitchen sinks and the vent for the basin. (F.Y.I. the basin doesnt vent to the outside, it has 2 studor vents.)

Might i add that the sink the water is coming out of has a s-trap vs. a p trap.

Boy! Someone was really not reading, or knowing, fluid dynamics or codes when they did your system, based on what you wrote.
1. An ejector basin will not work with an AAV
2. "S" traps are illegal, with or without an AAV.
3. AAVs require at least ONE vent, properly located, out of the roof in the system.

Either #2, or #3 could cause your problem and #1 might prevent the basement toilet from flushing properly.



Edited 2 times.

From reading around on the web, those were my guess, but needed a second opinion.

And, unfortunately, those are NOT the only possibilities, just the most likely ones.