Hi Verena,
difficult dilemma.
I think it would be OK if the people who are involved in the project, are aware and agree that you are going to present their work at the conference. As long as that is the case and you really want to go the conference, that it may be worthwhile. However, if it would be the opposite, if you feel that things are done behind people's back, if you have a bad feeling around the whole thing, I would keep my fingers off. I think it would be good to address these issues first before spending several weeks on the data analysis.
If you decide not to do it, be careful in how you present this to your supervisor, probably a good idea to be diplomatic regarding why you do not want to do it. Good luck!:-)