Why do people think the op's son's business is failing? He's only been doing it for a year, it can take longer than that. When my husband first went into business it took at least two years to make any money but it paid off in the end. It's quite brave to go into business after having a secure, well paid job - he didn't choose to be made redundant but as it happened, he took the opportunity to try self employment.
Another year and the business may be flourishing.
Also suggestions that the DIL trains as a teaching assistant are quite ridiculous. We don't know her, she may have absolutely no talent at all for teaching, it is a vocational job. Being stuck for hours at a time in a school, responsible for thirty children, is not everyone's idea of bliss. I wouldn't do it, would rather work in a launderette!
The OP certainly doesn't hate her daughter in law, she is helping her and her son. She just has misgivings about the woman being a stay at home mother when more income is needed.
I'd like to know why the daughter in law doesn't want to work: being out of the work force for so long could make the prospect of a job quite intimidating. She's probably scared and feels she can't do anything! For those of us who always worked, even with kids, it seems odd but not everyone has a lot of confidence.
OP, reduce your financial input because 50% of your income is too much, but I support you in helping your family financially as much as you can and would do the same. If the business takes off you will be rewarded in time.