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Great question, but my goodness... those boys have been through a lot. I would like to praise you at length for stepping in when mum was out of the picture - this was clearly a better situation for the boys. I could go on, but will answer your question directly.
In general, it is extremely upsetting and disorienting for children to have a parent step in and out of their lives. They grow confused when they have to say goodbye, and over time can grow quite resentful if they do not feel like a priority in the mind of the parent. But here is why I feel your situation may be different: If mum has been very consistent in her ringing them weekly (I would hope that as she becomes more stable, the rate of phone calls can increase as well), then she has been a constant ion the minds of the boys. In other words, it will not feel as if mum merely "pops up" after 2 years of absence.
I think that if she IS going to commit to this type of visitation, then it has to be regular and predictable for the boys. Being able to predict when they will see their mum will help them ease into the transition of letting her back into their life. If she is going to pop in and out at her leisure, then I would argue that she is seeing them for her own selfish reasons (and when they fit into her life) rather than for THEM and their emotional development.
It is a tricky situation, and one that should be avoided at all costs if mum shows any signs of relapsing and hurting the boys all over again. But for the time being, and as I mentioned before, if she is consistent and committed, then having a healthy mum part time is better than having no mum at all. For your part, you must prepare the boys for her visits and be ready to deal with sadness and probably some anger/acting out in the aftermath of the visit - it will take a good year or so for them to adjust to this new schedule and settle in.
I do wish you well. It is hard to see kids hurting, but careful preparation and allowing them to vent emotions can make this quite do-able. If you are satisfied with the response, please hit "Accept." That is the only way I can receive credit for my answer. Thanks-