You both have to sign the I-751.  They will reject the I-751 if you do not sign it.  You sign in Part 7, item 6.a.  Husband (US citizen) will sign Part 8, item 6.a.
 
	"If your petition is not signed, or if the signature is not valid, we will reject your petition."
 
	 
 
	Luckily, the instructions say this:
 
	 
 
	"Validity of Signatures. USCIS will consider a photocopied, faxed, or scanned copy of an original handwritten signature as valid for filing purposes. The photocopy, fax, or scan must be of the original document containing the handwritten ink signature"