I think I'm probably with you ahead of your sister on this, unless you also have a large amount of grazing you can also include in the price (or is available nearby), most people will not want to rent a stable without at least a couple of acres of grazing as well, so I doubt you'd earn much in exchange for the hassle. Do the stables have electricity/lighting, security features (cameras, padlockable gates etc) and running water installed as I think most people would want this too as a bare minimum? Also is there good hacking/bridleways near you or is it all busy roads (unless you have a surfaced arena or flat dry field to ride on you will have to look for people that only want to hack or that have retired/unridden horses and the latter aren't that common)?
Personally even if you say yes to all of the above, I'm still not sure I would want a bunch of strangers hanging around my home (I'm assuming the stables are in your garden) at all hours (in the winter they may need to be there very early to see to horses in daylight before work, in summer they might want to ride in the evenings), even if you strike lucky and get some lovely, quiet, very tidy people that look after their horses well and have no children or dogs to make extra mess and noise (and not all horse people meet this description by any stretch of the imagination!), there will have to be a muck heap somewhere which will need regular removal, there will be regular deliveries of hay and bedding, said hay and bedding gets everywhere if not swept and tidied very regularly, occupied stables can be a magnet for rats and mice, horses are prone to injuring themselves horribly or getting suddenly very sick at inopportune times (or is that just mine 
?) so you may have late night vet visits (and if you aren't horsey, having a horse colic or get cast in the middle of the night and having to try and deal with it while calling owners etc would be an absolute nightmare, you'd have to specify in the rental agreement that you take no responsibility for care of the animals of course but even so if there's a really sick horse on your hands you'd presumably be traumatized by it). Plus you'll have to pay business rates/tax on the income, sort out insurance and regular maintenance etc. Generally if you're not a horsey person I would think the whole thing would be way more trouble than it's worth.
I'd convert the stables to a garage or workshop or storage area or even a granny annex and let that out in preference to renting it to any kind of livestock TBH. Or you could keep other animals, chickens or other poultry could live in a converted stable happily enough, or let you DC use it as a glorified playroom/summerhouse. Or just knock them down/sell them cheap to someone on the condition they come and remove...
Of course like I say if you actually live on a farm or have loads of acres of unused grazing land it might be different but even then there is so much to think about and costs which will eat into your profits so even for farmers etc it's not that popular an option these days...