There is a lot of threads asking about this and the devs have said they are looking into it. So far, if it does happen, it will be just the base game for the steam version. So nothing yet. This is just my speculation but I feel the devs think its a sure thing BUT it be silly to say it without not knowing 100% themselves cause it be a real rug pull to say yes and then unable to deliver. 

Check steam forums for sins2 for more of its discussion but otherwise we should know in the coming week or so if its a ye or nay.

I would say it is now unlikely given how close release is with no word. I will probably just buy it on steam to get away from epic. None of my friends will see me pop up playing it on epic. 
The question for me now though is if I miss any goodies by making a clean break from epic?