I've been cashing checks for mom for her personal spending and signing the backs. Is that going to look like I received money from her?

You already know it looks bad or you wouldn't be asking. How long has this been going on? At $500/mo plus bill payments it is going to look as though you are making out like a bandit, not only to siblings but at Medicaid if she needs to apply within the next 5 years. I would write up an itemized accounting with receipts and get her to sign it, then look at alternatives going forward, auto pay the bills or have her sign checks for bill payments that you can fill out as needed. I hesitate to suggest having her use plastic instead of cash because so many people run into trouble that way, but the cash withdrawals are going to be the hardest to account for so you need to be extra careful.

Compile all those bills of hers that you paid and show reimbursement that you received. Five hundred a month is hardly making out like a bandit and should not be too difficult to itemize. Set mom up for auto bill pay for what you have been paying, then move forward.

Sorry for the "making out like a bandit comment", I meant that statement as what other family members may believe, especially since there appears to be no caregiving involved.