I am assuming the fixtures on the first floor drain down this main waste stack, So your toilet and sink can not use the waste stack as a vent. The toilet will need to be vented before it connects with the stack, you can drain your sink into that vent. Then that vent needs to go up above the fixtures on the first floor (42" min. above first floor) before reconnecting to an existing vent stack. The basement sink is close enough to the toilet that you do not need to revent it if you drain it into the toilet vent.