Laundry Room Remodel - Drain Trouble

Hello all,

I've been searching online trying to find answers to my reno questions, and constantly came up with hits from this forum, so I thought I should sign up.

I am redoing the laundry room and would like to replace a double laundry sink with a single, add an outlet/connection box for the washer (supply lines and drain in wall, nice and tidy) as well as a drain for my new water softener.

The old laundry sink and washer standpipes are 1 1/2", were not vented, nor did they have P-traps (didn't really notice a smell, and I am on a septic tank/field; not sure if that makes a difference). There is a 3" soil stack running from the upstairs 1/2 bath that I was planning on tapping into, but then read about NOT draining downstream from a toilet.

How should I go about connecting my washer standpipe, single laundry sink, and water softener drain?
Can the softener drain into the laundry sink, or does it need it's own standpipe?
Will I have to break open the floor and replace the 1 1/2" pipe with 2"?
What about venting this mess (I am in Canada, if that makes any difference)?

Hopefully the pics attached properly, so you can see what I'm dealing with.

Thanks

1-1/2" is fine. you would need to vent it to the floor above and tie into existing vents above flood level rim of uppermost fixture...there may be a trap below the floor serving that line....not good practice, but it may have been done that way...the softener must be an indirect connection so draining into the sink is a good option

Pretty strange setup.
In the US, 2" would be required for the washer standpipe, and the p-trap would need to be above the floor level.
The vent would be 1.5" tied in at 42" above the next floor.

The washer and the laundry tray would revent to each other at 42" above the basement floor.
Both the washer and the laundry tray would have p-traps.

You are out of your league top make THAT kludge work with a new laundry sink, washing machine, and softener drain.

Newer WM tend to have higher velocity pumps than the old ones, and you might have problems with a 1.5" line...depends. In the USA, code requires it to be 2", Canada may be different, but physics aren't!