Adding laundry sink drain plumbing to washing machine drain pipe?

I want to put a utility sink in the basement with a tank and pump to discharge overhead. Almost exactly above the sink location in the basement is our washing machine upstairs. It discharges through a rubber hose with a 180 degree bend in the end, which goes into a PVC pipe that extends upward about 4’ from the floor (conveniently, it’s not buried in the wall).

I hope I can replace part of this pipe with a larger diameter into which I can put both the existing washing machine drain and a new pipe from the sink downstairs. Certainly, this is physically possible, there’s room for a larger pipe and easy access for cutting.

Just wondering if there is anything weird that could go wrong. For example, if the present pipe (I think it’s 1.5" nominal) is big enough for the washer, would 3" pipe with a reducer to 1.5" be OK? Or would the reducer be enough of an obstruction to threaten overflows?

Thanks!

Are you fairly certain that only one would be draining at a time?

One thing to consider: for the washing machine, does the 4’ of exposed drain ever surcharge? You’d want the entrance of the new drain to be above any surcharge elevation. Otherwise you’d get upstairs drainwater backflowing to your sink drain.

If you can remove the existing washer drain and trap. do so. Add a “T” in its place. Connect a trap to one side of the “T” and reconnect the washer drain to the trap. Add a trap to your sink connect that trap to sump tank. On the discharge from the tank connect a good check valve then a gate or ball valve. Then connect the line from the valve to the other side of the “T”. the tank is also going to need a vent line ran from the tank to outside or connect it to an existing vent line.