if it is feasible, a safer way (for her and your back) might be a patient lift? does she have either medicare or medicaid coverage or a private medical policy with durable medical coverage? if so, they would pay to rent or buy a lift. read here for more info. on patient lifts:
http://www.just-patient-lifters.com/
check out the chart at the bottom of this page
http://www.spinlife.com/category.cfm?categoryID=108
http://www.google.com/search?sourcei...=patient+lifts
i have next to no leg strength but i can still use the type of lift (sit/stand lift) that is primarily only for people who can bear weight, which is a bit less time consuming to use than the hoyer type lifts. both have advantages/disadvantages.
then thery make manual ones or battery operated ones that require no exertion by your back. there are also the kind that attach to the studs in the ceiling.
i found a slightly used portable lift for $1,000, orig cost was $4,000. it has been a lifesaver for my husband's back.
i know you weren't inquiring about mechanical means of lifting her but this is just a 'fyi' for the future, if need be.