they can claim you if these 3 tests are met
1) the relationship test is met - this seems to be the case
2) your gross income for 2019 less than $4,200
3) they provide over ½ your support in 2019
here is a link to a worksheet to determine if they did or didn't provide over 1/2 your support.
https://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf
if they didn't you can report that they falsely claimed an exemption for you on form 3949-A
or even if your gross income was over $4200.
Gross income means all income the person received in the form of money, goods, property and services, that isn’t exempt from tax. Don’t include social security benefits unless the person is married filing a separate return and lived with their spouse at any time during the tax year or if 1/2 the social security benefits plus their other gross income and tax exempt interest is more than $25,000 ($32,000 if MFJ).
https://www.irs.gov/pub/irs-pdf/f3949a.pdf
