Takin on tasks

I have a situation where we are trying to get some legal paperwork completed and filed for my in-laws. We took on part of the process and another relative was incharge on completeing the other part. Now as we come close to the completion, the other relative is showing up with incompletely filled forms, missing details and lots of small tasks that need to be completed before submission. This relative lives in a diffrent city and during their recent visit, he dumped all of the incomplete paper work on us and said that he will call us with the information needed to fill it in because they are too busy to do this. I am suffering and caught between wanting to help my in-laws whilst feeling like a doormat who is being dumped with all these loose ends because someone else thinks their time is more valuable. So i did a download
C- Paperwork for In- laws
T- They are dumping taks on me
F- Resentful
A- Do the tasks anyway because it is important and time sensitive
R- give away my power of choice and my own time management
T- I have complete control over my time and all tasks that I choose to do
F- Empowered
A- Set aside 1 hour- finish some of the loose ends that i can complete, tell them they are in charge of the rest
R- Task is done, I held my own power.
Im struggling to beleive this model for 2 reasons- 1. If i get too caught up in the division of labor, my in-laws are the one’s who will suffer,I don’t want to do that to elderly people- seems so unkind. 2. I do know, that irrespective of how I show up, the chances of this relative completing their part of paperwork is really 0%. How do I assert myself/just do it anyway but feel better about it? Please help. Also PS- I have spent way too much time seething about this- I suspect that it would have atually taken me lesser time to get the stuff done than the time I have spent suffering about it!!