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The part of the letter that scares me is that the daughter is paying payments on a variable-rate interest only loan. Not only is the daugher paying more each year for this loan, but the mother is getting less in equity, and less to pass on to her children and church in her will.
It could be argued that by paying a 10% tithing now, and going into debt doing it, equates to the mother not being able to will anything to the church when she passes.
The daughter should also realize, that by paying the mortgage, she is also generating future income for her mother should the mother choose to get a reverse mortgage as a source of income. So not only would the mother be using the equivalent sum of mortgage money to tithe now, but she would be using the actual equity that the daughter purchased to tithe later.
In conclusion, the mother should get credit counseling, and also seek counseling from the church on non-monetary tithing. The daughter should can still help with the mortgage by paying of principal parts of the mortgage for her mother. This will force the mother to still make the interest payment portion, but assure that the mother is building equity in the home, living within her means, and using her new skills acquired from credit counseling.