1. You do not include any of her SSDI or the lump sum as income on your return. A taxpayer does not include his dependent's income. The dependent files her own return, if she has sufficient income. If SSDI (including lump sums) is her only income, she does not have to file.
2. SS, including SSDI, doesn't count as income, for the $4050 income test,
but social security money he/she spends on her self does count as support not
provided by you, for the support test. Money she puts into savings &
investment does not count as support she spent on herself. If no one person provides 50% of the support (for example her family is also sending
support), then a "multiple support agreement” (IRS Form 2120) can be used,
to allow you to claim the dependent. https://www.irs.gov/pub/irs-pdf/f2120.pdf
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.
3. The lump payment is only considered support if it is spent on support. It isn't support if it goes into savings.
The being older rule does not apply to her. It's for claiming a person who can be classified as a "Qualifying Child". Since she is not related, it's not relevant.
