In my area, the local thrift stores will NOT take used baby gear; strollers, crib, highchairs, bouncers, etc. There have been too many safety recalls. They don't want the responsibilty. If it's not new in the box, they won't take it.
I'd take SIL's stuff, tell her I'll be selling/trading what I don't keep and then put that money towards baby stuff I'd want. That is what I did with all the baby stuff given to me.