moving toilet and sink drains

I am in the middle of a remodel and i have cut the slab up and want to move the toilet 4' to the left and the sink drain 3' to the right. I have cut the cast iron 4" toilet drain and i am putting a fernco fitting on with a 90 to the left then 3 1/2" of ABS then a 90 up to the floor flange. measurement from studs to bolts on flange need to be 12 1/2" correct.

as it is now the toilet drain transitions from 4" cast iron to a 2" cast iron pipe for the vent. there is about 6" of ABS between the two. can i cut into that and put my sink drain right into that? Right now sink drain is on the opposite side of the toilet main from where it is moving to. The shower and sink vents tie in with the toilet vent about 5' from floor now.

photo of the pipes

here you can see the main toilet pipe with the sink and tub vents coming in I want to cut into the bottom by the ABS pipe