Academics usually have a written set of duties to their students under university rules. This falls short of a fiduciary duty to the student, but it usually entails duties to facilitate the progression of their candidature. It is perfectly acceptable for a PhD student to work on other research outside their PhD topic from time to time, but the supervisor should not assign work to the student that is at odds with progression of the candidature or advancement of their academic progress. In view of this, the work that your supervisors ask you to perform should generally fall into one of the following categories:
Research, administration, and other work that is related to your PhD candidature;
Coursework or other learning activities that are properly connected to your PhD candidature;
So long as it does not hinder your ability to progress your candidature, research work that you have voluntarily accepted and which is unpaid, but gives you some valuable academic benefit (e.g., co-authorship of publications, your name as a researcher on successful grants, etc.);
So long as it does not hinder your ability to progress your candidature, research work that you have voluntarily accepted and which is paid (and even then you should get appropriate credit for your contribution).
From what you have described, a small amount of the work that you are doing is paid, but most of it is unpaid. Your supervisor should only be asking you to do this work if they judge that it will give you some academic benefit (albeit one that might not line up with your PhD research topic) and they judge that it will not hamper your progression in your PhD candidature. If you have concerns about this, you should raise these with your supervisor, and ask for an explanation of how this activity will advance your academic development, and how you can balance it with your PhD research.
Depending on what has been arranged, you should get some academic benefit from this work, due to being listed as an applicant on an academic grant, or as a co-author of publications. If this is not the case, you should not be doing the work. Since you are undertaking lab work for a grant application, you should ultimately get credit for this, through co-authorship on publications that use your lab work, and possibly also being listed as a co-applicant (or at least given an acknowledgement) in the grant application. You should make sure to negotiate this up-front --- have a talk with your supervisor about expectations for co-authorship, etc., to see what credit you will be given for your contribution to this research. Assuming you are given appropriate credit for your work, it might be a valuable addition to your accomplishments during your candidature.
In any case, in the first instance, I would suggest you raise all these concerns with your supervisor. Make sure to have a discussion about what credit you will get for your work, in terms of co-authorship of papers (authorship order, etc.), and whether you will be listed as one of the applicants on the grant application. You should also discuss timelines for your PhD candidature, and make sure your supervisor is giving you time to progress your actual topic. If you are unsatisfied with your supervisor's plans and responses on these issues, raise the matter with the relevant graduate-student co-ordinator and ask for a second opinion. Also note that, if you are not getting appropriate academic credit for this work, and it conflicts with your PhD research, it should not be assigned to you.