Did your parents do a Current Year Income assessment for your Academic Year 14/15 because their income had dropped since tax year 12/13? That would explain it.
If your parents did a CYI for 14/15, then:
for AY 15/16, they would want figures for tax year 13/14 as normal (even if your parents do another CYI for 15/16, they would still want the 13/14 tax year figures)
for AY 14/15, they will need evidence of 14/15 tax year income to confirm the CYI assessment that was performed when you applied for first year (it is initially based on estimates because the current tax year is not finished)
If the CYI evidence is not confirmed within a certain time, then the application reverts back to the tax year that has confirmed figures (for AY 14/15 this would be tax year 12/13). If your parents' income was too high in tax year 12/13 for Maintenance Grant then this would cause it to be removed. As you're in-course, then any grant overpayment is taken off the subsequent academic year, so you end up receiving two entitlement letters with new amounts for both years.
Assuming I'm correct about the CYI (and I really don't know what else would cause this):
1) The reason for your 15/16 entitlement being reduced is an overpayment from 14/15 - they haven't messed with your AY 15/16 application but the overpayment from AY 14/15 has changed the entitlement.
2) They're not ignoring the evidence. You're getting confused between evidence for AY 15/16 and the Current Year Income Confirmation for AY 14/15.
3) This should cause an overpayment on your sister's account as well, unless she has actually been assessed as an independent student and your parents' income is actually having no affect on her account. Is she older than you? (Also, technically, your account is not linked to your sister's... both accounts are linked to your parents' accounts.)
As for the different letterheads, Student Finance England and the Student Loans Company are actually the same entity.
The person your dad spoke to on the phone four weeks ago was wrong. The letter he will have received will have been a Current Year Income confirmation letter, not a scam. The person he was speaking to on the phone probably couldn't see it because they were looking at your account instead of his account. Your parents need to return the Current Year Income confirmation with evidence, then your AY 14/15 will be reassessed to remove the overpayment.