You should write the current income projection that your SIL can provide documentary evidence for. Example, if she has a recent paystub and that paystub shows that she gets paid every 2 weeks, then the calculation should be -- gross pay from that paystub x 26. Submit a scan of the paystub in CEAC as evidence of current income. Although not required, you may also put a note in the Additional Information section of the I-864 to explain the income calculation, something like "current annual income = gross pay from May 19th paystub x 26 pay periods in 12 months".