Developed technique but was not added as co-author
The proper advice, I think, is to ask for co-authorship on any future papers that use your technique. Point out that you are actually due this for having developed the technique and even more for adapting it to the other students' work. Not for "helping them" adapt it. For adapting it. Nor would I be shy about saying that they made an error in not including you on the recent paper. 

They might just offer an acknowledgement and you can decide if that is enough for you. I don't have a recommendation about that, but think that co-authorship is probably more appropriate. 

And, if they say no, then your decision about helping is also pretty clear, I think. 

I wouldn't make the suggestion quite so strong except that you say the relationship with your advisor is already toxic. You have your own career to think about and your own research goals and needs. You don't need to also do someone else's work without proper attribution.