Someone said it would be preferable to have her sleep on your couch until she had a certain number of months' rent money accumulated -- Please don't! You will have great difficulty getting her out of your home if you allow her to stay there at all, from what you describe. She will find it so easy to be in someone else's house as a "guest" compared to being responsible for a house on her own. Whatever happens with the rental home, don't make your own home an option for her for more than a night or two, with her suitcase close by and packed, or you will have a bigger "pickle."
I'm concerned (for her but also for you and your husband) that she apparently has a formal diagnosis of depression but it's not clear if she is under any regular care. Maybe you know these answers already, but: Does she see a therapist or other doctor on a regular schedule? Does she maintain that schedule no matter what else goes on in her life? Is she on medications, and can she afford them, and does she actually TAKE them as prescribed? Does her therapist, if there is one, know that she wants to make this move? If she moves, she must have another therapist lined up to see her regularly in her new location. (A friend who has moved several times and who has mental health issues is always careful to find a new therapist immediately when coming to a new place, so she can stay on track mentally and emotionally -- has your SIL even thought of this, I wonder?). She should not move until she has talked this out with her therapist, or someone professional. You may be reluctant to ask her all this but your husband -- who, I agree with other posters, should be the main point of contact with her -- should tell her that with her depression diagnosis, she needs to get therapy and meds in order before making ANY move. Of course, she could simply lie to him and say that's all under control, she's taking her meds, she's got a regular therapist who's delighted with her plans, etc., but you have to at least ask. If she gets to this new city and falls apart mentally and emotionally because of the total lack of support -- or enabling -- your husband at least will have made the attempt to have her see that her mental health should come first.
You mention that she might need "a life coach, a social worker" etc. but it sounds like she needs a therapist, now. Then you might be able to put her in touch with a "job coach" or head-hunter firm or employment agency rather than a life coach, in the city where the jobs are. I bet her lack of work over decades is going to sink her chances of getting any job even in that job-rich city, but let a head-hunter or employment agency be the ones to tell her that -- BEFORE she makes the permanent move there. She should be doing serious job-hunting trips there and she should not stay in your property while doing those trips or you may find it to tough to get her out if she doesn't land a job.
I think you are right to tell her she must have a job before she moves into your property. Frankly I would not let her live in my property because I've seen nothing but rifts and hurt feelings come from families doing business (including rental business) together, but if you want to let her live there on the generous terms you have offered, that's great. Just stick to your guns about her having a job -- NOT just an offer or an interview but a real starting date and a written contract she lets you see -- before she moves in.