So there’s two things here:
1) Can you translate / use others to help with translating.
2) Should you have failed your placement.
To answer 1, it’s not ideal to have either yourself or a relative as an example translate, but it’s not illegal or strictly against policy. It’s very common on the wards for instance to use a member of staff who speaks a language to translate where no interpreter is available, much as you have experienced. For explaining weight bearing status as an example, I wouldn’t be concerned about using someone to translate. If you were discussing financial power of attourney with a family then that would be another matter as a patient could be open to abuse with a family member / member of staff telling you lies such as I want to leave all of my house to the healthcare assistant on ward 3b. As with many policies, there is a reasonably large grey area.
In terms of being unfair to complete the session, I suspect the answer is no. If you as the clinician felt things were unsafe then you should have stopped. If the patient wasn’t following instructions or went to do something silly then the expectation is you would physically guide them - a hand on the chest to stop them standing, a lift of the affected leg to put it in position etc. It’s difficult from what you’ve written to understand exactly what went on, but it seems that the patient has gone to stand up and your educator has jumped in and stopped them? For me personally as an educator I wouldn’t have failed you on this, but is there more to the story here?