Can My Sister Sign Mom's Will as Her POA?

The issue is not just that your sister can’t sign as POA. It’s that no one can sign a will on your mother’s behalf, POA or not. There’s also a second issue worth addressing here.

Why a Power of Attorney Can’t Be Used to Sign a Will

A will is considered a deeply personal act. The law requires that the person making the will (the “testator”) have and exercise their own testamentary intent at the moment of signing — this authority cannot be delegated to an agent, even one with broad POA powers. So your instinct is right: your sister, acting as POA, legally cannot sign a will for your mother, no matter how the document is worded or how clearly your mom's wishes are documented elsewhere.

A small number of states allow someone to physically guide the testator's hand or sign at the testator's direction if she’s physically unable to sign herself — but that’s different from a POA standing in for her. It only works if your mother is present, has capacity, and is directing the signing herself in that moment.

The Part That Needs More Attention: Her Capacity

Since you mentioned declining cognitive health, the more pressing question isn't who signs; it’s whether your mother currently has testamentary capacity. This is generally a lower bar than the capacity needed for financial or contractual decisions, but she still needs to be able to understand, at the time she signs:

• That she’s making a will
• Roughly what she owns
• Who her natural heirs are (you and your sister)
• How this document distributes her property

Cognitive decline doesn’t automatically disqualify someone — capacity can fluctuate, and many people retain testamentary capacity well into a diagnosis like dementia. But it does mean timing and documentation matter a great deal, and it's worth acting sooner rather than later if she’s still able to express clear, consistent wishes.

Why It’s Safer If Neither of You Drafts or Directs This

Since you and your sister are both the intended beneficiaries, and your sister also holds her financial power of attorney, it’s wise to keep some distance between the two of you and the actual drafting process — not because anything is being done wrong, but because it protects the will (and your mother) from a future challenge.

If either of you selects the attorney, sits in on the planning meeting, explains what mom wants, or otherwise directs the process, it can create an opening for someone later to argue undue influence — especially given her cognitive decline. Wills involving a POA-agent-turned-beneficiary tend to draw more scrutiny if contested.

1. Hire an independent estate planning attorney, meaning independent from you and your sister, not from your mother. If your mother already has her own longstanding estate planning attorney, that’s usually a good choice, since a documented history with her supports her testamentary intent. The key is that neither you nor your sister selects the attorney, arranges the meeting, or attends the part where she confirms her own wishes.
    
2. Let the attorney meet with her alone, at least for the part where she confirms her own wishes and the attorney assesses her capacity. This creates a much stronger record that the will reflects her intent, not anyone else’s.
    
3. Consider a contemporaneous capacity evaluation from her physician, especially given the cognitive decline. This isn’t legally required in most states, but it’s one of the most effective ways to protect the will from a future contest.
    
4. Act with appropriate timeliness. If her cognitive health is declining, waiting increases the risk that a future capacity challenge becomes harder to overcome — but rushing without an independent process creates its own risks. Get the attorney meeting scheduled soon.

Bottom Line

Your sister’s POA doesn’t just fall short of letting her sign the will; it’s a completely separate legal act that only your mother can perform herself, with capacity, before an independent witness (typically her own attorney). Given her health and the fact that you and your sister are the intended beneficiaries, looping in an independent attorney now — rather than either of you shepherding the process — is the safest path to make sure her wishes hold up.