What seems missing from this post is what is she doing to support herself , as Thai men (if two different fathers) will be unlikely to pay enough child support for her not to work and child benefit here seems very little. Who takes care of the kids if she is working, what does she work at ?
You should NOT be paying anything in sinsod, she is too used for this money. I paid zero, she had 2 good kids and good family but far too old and used - don;t like saying that but its true
If you feel the need to give her FAMILY some monetary help then that is OK, but spread it out over time in the form of support and not a lump sum.
I also fail to see from your post how you would live with her, you do not appear to have a job to work here and can't stay here without work.