Need a vent in the shower drain?

Adding a shower to 1/2 bath in basement

I will add a shower to the 1/2 bathroom in our basement. right now I have a toilet (end of the drain line) that drains into a 4" pipe, 30 inches down the line the sink drains through a 2" pipe and has a 2" vent (work as a wet vent for the toilet). The shower will be after the toilet (3 feet from shower drain to toilet drain, 6 feet to the sink drain-vent). I was planning to attach the 2" shower drain to the 90 of the toilet, Is this OK?. Do I need to vent the shower or the sink vent will work? In case that I need a vent, what will be the best way to do it? Putting the new vent betwen the shower drain and the toilet drain?
Thanks in advance
Frank

uh-huh. No problem in my mind, yet. But I'm not a plumber...

A drawing would help attract viewers, and explain it all at once. I had to re-read your post several times.

david

You kinda lost me. To the best of my understanding you have ONE vent for the sink and toilet and want to use that same 2" vent for the shower? If yes, then I think it's a bad idea.

Also, if you are planning to run the shower line into the toilet's drain path before the toilet is vented, and if there is no vent between the shower trap and the toilet flange, your shower might get siphoned during a flush.

Good venting rules of thumb:
1) Every fixture gets its own vent
2) Every vent should take off a foot or two after the fixture's trap (with the possible exception of the toilet).

Thanks. Specialy for understanding my terrible description of the situation.
I will put a 2" vent between the shower trap and the toilet flange and connect this vent to the sink vent up higher (2' above the sink).
Is this OK?