an adopted child is treated as a child by blood
thus your sister would qualify as your mom's qualifying child if all these test are met
- she have the same principal abode as mom for more than ½ the tax year. Temporary absences like for school are ignored
- she be under 19 as of 12/31/2018
- mom has provided over ½ her support
- no joint return unless no tax liability but merely filing joint to facilitate refund of taxes withheld or estimates paid
can say how much tax benefit mom would get if she amended .18 to claim your sister
any dependent older than 17 is not a qualifying dependent for the stimulus check.
