How to write a statement of research interest for postdoc positions that already have a defineed research direction
One way is to write an SOP that is a bit more general then you would do otherwise, but not so general as to seem unserious. If you do it carefully, it can be an advantage. I think that an SOP too tailored to a particular position might not be taken seriously -- too good to be true. But don't go to the other extreme of suggesting that your interests are so broad that you don't/can't specialize as needed. 

I wouldn't recommend this for positions that were widely different, but it might be good here, as you say they are related. 

But, as you say, the direction of the lab is set and a post-doc isn't likely to change that. If your research interests are aligned, your "future research" ideas will probably fit as long as you don't overemphasize the specifics more than needed. 

This is a Three Bears sort of problem. Too general. Too specific. No, just right.