When showering and/or flushing the toilet upstairs my kitchen sink fills up and over flows... I have tried different drain cleaners to no avail... please help

That indicates that upstairs bathroom is connected to kitchen sink drain in one common drain. That common drain is clogged somewhere under past the kitchen sink, somewhere under the house. There should be a clean out behind or under the sink. Use 3/8" snake and snake it through the clean out. Flush for few minutes with hot water.

Hope that helps

Milo