A few weeks ago our kitchen sink started backing up with water whenever we use the washing machine. We are on a septic system and have never had any problems until now. The house was built back in the 50's so we're not sure if there is a grease trap to be the cause of the issue. We've put root killer in the crock, but that didn't help and we just used an electric auger to try to clean out the pipes. Nothing has worked and we are very frustrated that it might be a larger tank issue. Any suggestions would be great.

There are several ways to check your septic tank. The obvious would be to open a inspection cover on the tank and the other would be to to open up the house clean out and cycle a few loads of laundry. If the cleanout backs up you need tom have the tank ckecked to see if it needs pumping.
If not then the blockage is downstream from the kitchen sink and Iwould go on the roof and snake from the kitchen vent pipe. Put out enough cable to reach the base of the stack and about 25' more. Good luck, Tom

Unfortunately we've already done everything you've suggested. Last year we had the septic tank pumped. We did notice that the kitchen sink did not drain into that tank. So we assume that we have a second tank, or something, under the ground that the kitchen and laundry is draining into. Looks like we'll have to dig up the back yard to try to find out where, and what is going on anyway. Thx for your reply!

It sounds like you have the same set up that I have. The washer and kitchen sink discharge into a grease trap and the liquid goes from there into the septic tank or direct to the drain field. This is the best arrangement for a septic tank that there is, especially if you have a garbage disposal. The grease trap,( interceptor) traps the fiber from the washer and the grease and crud from the sink and prevents it from passing through the septic tank and clogging up the drain field. The grease trap is located near the sink so if you can find the drain pipe track it back and clean the grease trap. You may have to snake the kitchen vent out also, but I'm betting that when you clean out the grease trap your problem will be solved. Good luck, Tom