How can I help my mother?

(Posted this in GENERAL, but have yet to receive any input; hopefully this is an appropriate place to also pose this question, as my mother (amd my siblings and myself) have been long-time sufferers of my father's emotional abuse)
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Hello, all,

I'm not particularly new to this forum (as I suffer from mental health issues of my own--however, that's not why I'm posting here). I'm really concerned about my mother (with whom I do not live at the moment--was kicked out of my home by my father for really petty reasons, in my opinion), as I'm been made aware of the fact that she's again abusing opiates and is downward spiraling into a massively depressive spell. According to my two younger sisters (who are currently in my parents' household), my mom spends the majority of the day sleeping, often nearly falls asleep at the wheel while driving, has gained a relatively significant amount of weight (which in and itself isn't a problem--but my mom has always micro-managed her weight and appearance, so for her to suddenly become disinterested in these things is really out of character), has made suicidal comments (if she had a gun..., etc., etc.), and has more or less lost all interest and any real ability to conduct herself within the context of daily life. I should mention that she's struggled with depression and opiate misuse in the past - and indeed did overdose back in 2009, at which point she was hospitalized and was semi-serious about receiving treatment in an intensive outpatient program - but nonetheless, I suspect that she might be using again. In addition, her mother (my grandmother) passed away back in late March of this year (she suffered from Alzheimer's disease for the past ten years or so, and I know that my mom felt tremendous guilt in placing her into a home)--and since then, she has seemingly grown increasingly detached and hopeless (she didn't even really grieve the loss of her mother, which I found rather odd...).

While a direct approach would be best suited, my dad is...well...completely and utterly unreasonable, and has been emotionally/mentally (and sometimes physically) abusive towards my mom, my siblings, and myself ever since I can remember. He's had a pretty massive drinking problem in the past (continues to drink lightly every night, but not to the point of making him abusive--although he does heckle and control my mom (what time she goes to bed, when she goes to the grocery store, etc., etc.)), and is (I suspect, anyway) the male manifestation of borderline personality disorder (although he'd never admit it, and remains completely and utterly self-unaware and unwilling to take ownership of his behavior). He's acutally told my siblings and myself that he think that I've been trying to "brainwash" everyone into "turning against" him, and rarely (if ever) leaves my mom alone long enough for me to even have a conversation with her. At this point (from what I gather), he's really been breathing down my mom's neck, and has threatened to kick her out if she overdoses once more (which makes me completely irate considering all of the nonsensical and toxic BS he's put our entire family through)...

I am deeply worried about my mom, and feel disempowered as far as helping her in any meaningful capacity is concerned. I want to try texting her (she NEVER answers her phone or responds to texts, but I suspect she does read them--unless my dad also micromanages her cell phone, which honestly wouldn't surprise me at all...). Does anyone know what I should do? Please, I could really use any thoughtful input and/or advice. Thanks in advance.