You may want to discuss with your husband conditions that would need to be met if he decides to allow his mom to live with you. These conditions would apply to any adult family member that wants to live with you (there are a lot of websites dealing with young adults that don't want to leave their parents house and I think these may help you come up with the conditions).
Like first, she has to have a job; even if it's just part-time (it might be better to make this a requirement before she moves in, and she has to keep it). Maybe you and your husband can sit down and work out a plan for her. Require that she contributes to the household....financially and/or by helping with housework, etc. Let her know that living with you is just temporary until she gets on her feet. And include a time limit for that to occur.
She may never become independent but at least you can say you tried to help without it destroying your own family. Also, there is a lot of internet help on how to deal with verbal abusers; which she is doing if she's using guilt to manipulate you and she's encouraging dissension and hostility between you and your husband.
Focus on the Family has many resources...Dr. Dobson's teaching on tough love is really good!
I think the most important thing is that you work to not allow this to come between you and your husband. Try to talk to each other out of mutual concern without resentment. Hard but possible! Pray for the Holy Spirit to show you how
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