I-864W and I-864

[Misia]

I am gathering docs to apply for my mother's green card. She has 40 credits from Social Security (survivor's benefits based on my father's work). Instruction to Form I-864W say that in that case she does not need I-864. The only problem I see is that her soc. sec is very small ($420 when she is outside US and $610 when in US due to taxes taken out when she is out) and does not met poverty guidelines which for 1 person are 100%-$10,210, 125%-$12,763, and will be little higher starting April 1st. The instructions to I-864W don't mention poverty guidelines but say that officer will determine ability to support self or someting to that regard. I am a nursing sutdent still at school (I will be the primary sponsor), my husband lost his job due to surgery, and we have 2 little kids, so we basically don't have income except what we have in investments, but not enough to sponsor her. Do we need to find a supplementary sponsor or she will be OK with her 40 credits. Do they take into account how much the benefits are or just that she has 40 credits. The sponsor that we have in mind is retired but has sizable investment income so can support his wife and my mother-whould that be a problem. In summery-her income low, we have no work income, sponsor has only investment income-- Any ideas????, Do we look for a different sponsor?