We have a scenario where a mom, her son, and his wife all 3 are owners of the principal dwelling of the son and his wife. Mom does not live there. Son is applying for a refinance, with mom as a co-signer. Wife is not on the loan at all. I know that son and his wife both receive a right to rescind, but does mom - since it's not her principal dwelling?

Mom - no. Not under Regulation Z.

The long answer's in Reg Z's definitions. Here's the rule:
In a credit transaction in which a security interest is or will be retained or acquired in a consumer's principal dwelling, each consumer whose ownership interest is or will be subject to the security interest shall have the right to rescind the transaction

Checking the definitions in Section 1026.2, here's how it shakes out:
1. this is a "credit transaction."
2. you will acquire or retain a "security interest" (the deed of trust).
3. son is a "consumer" (because the credit is extended to him as obligor), and daughter-in-law is also a "consumer" (by virtue of OI #2 to Section 1026.2(a)(11); mom may be a "consumer" (if she is a joint maker of the note), however if she's only a guarantor, then OI #1 says she is not a "consumer."
4. the structure is a "principal dwelling" (for son and daughter in law), but not for mom.
5. if mom is only on-board as a guarantor (no ownership interest in the property), then she is not entitled to the ROR.
6. if mom IS a joint owner of the property, but it is not her "principal dwelling", then she is not entitled to the ROR.

Bottom line: the only way mom could be entitled to the ROR would be if she's on the deed AND it's her "principal dwelling."

Mom is not a co-maker. Thanks all; this is what I thought; but for some reason our software is trying to give it to her. I'll look into this further in the system and see if there's any settings missed somewhere that could be causing it to populate in error,

Your system may not understand that while mom is an owner and a borrower; she doesn't reside there.

Yes, i'm guessing it probably comes from the way you enter the collateral...you only have one chance (at least in my LOS) to indicate whether the subject collateral is the "primary residence" or something else, and many LOS's can't then distinguish that the collateral IS the primary residence of one of the borrowers....but not the other.

Looks like you have two instant solutions:
1. Go ahead and give mom the ROR, even though you don't have to. If she's willing to assume responsibility without receiving the proceeds, she's not going to turn around and torpedo the deal 3 days later.
2. Let the system print the unnecessary docs and handle filing or disposal of the them by procedure.

At your convenience, you can look for any not-so-obvious settings that will force the LOS to spit out the right docs, or hassle the vendor to correct this flaw in the software (unless they want to bill you for a custom change.)

I think for now we will go with 'option 2' - I have other issues I'd like the vendor to fix more!!