The question is whether she can claim you as a dependent not whether she did. If, in fact, she can claim you then you must on your return check the box saying that someone else can claim you. In that case you would not be able to get the stimulus payment if that is what you wanted. 

What I am asking is clarification on the 2nd part of the question, unearned income? I only receive SSDI so I have no earned income, but this year  if my social security is counted for half of my unearned support , it is close.  I have heard it both ways , So say I pay more than half my support this year while her income has been in the toilet, what then? Does that make sense.

Ok, so if this year 2021 she starts working again, which she will I'm sure she will be able to claim me again without any problem. It just doesn't take too much to outdo SSDI and I don't want her getting flagged like she was trying to do something wrong. 

I own our home, but it is very modest.

Does this situation ever happen as I have had a hard time finding it, I am just chalking it up to the pandemic......

Q. Does this situation (I'm a dependent for 2019 and 2021, but not 2020)  ever happen as I have had a hard time finding it, I am just chalking it up to the pandemic.

A.  Yes. It's very rare. But, the circumstances you describe would account for it.

There are two types of dependents, "Qualifying Children"(QC) and standard ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). A person can still be a Qualifying relative dependent, if not a Qualifying Child, if he meets the 6 tests for claiming a dependent:

1. Closely Related OR live with the taxpayer ALL year
2. His/her gross taxable income for the year must be less than $4300 (2020).
3. The taxpayer must have provided more than 1/2 his support
4. He must be a US citizen or resident of the US, Canada or Mexico
5. He must not file a joint return with his spouse or be claiming a dependent of his own
6. He must not be the qualifying child of another taxpayer

Nontaxable Social security (including SSDI) doesn't count as income, for the income test, but social security money he/she spends on her self does count as support not provided by the taxpayer, for the support test. Money she puts into savings & investment does not count as support she spent on herself. Note that a parent is closely related so there is no requirement that she live with you at any time, during the year. But if you provided a home it helps your support case, unless they own the home you live in. 

The IRS has a worksheet that can be used to help with the support calculation. See: http://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf The support value of a home is the fair market rental value, divided by the number of occupants.