Just a note before I forget, you should keep that sink line capped, you're letting sewer gasses into your home.
We can't tell from the picture where the main stack is, which has to be functioning as the toilet vent in this setup. If its too far away from the toilet, or if the sink/lav drains tie into it wrong underground, you could have issues. There's not really any way to know that though.
Make sure there's no trap under the concrete on that sink drain. It would be stupid, but you'll find things like that sometimes. dump a bunch of water in there, then look down it with a flashlight. if you see water, there's either a trap, or a clogged pipe down there. If no water, you should be good to go, and install your standard lav P-trap off the riser to the vent, and tie the shower vent over to the lav vent at least 42" above the floor.