Cutecat78 I am curious about a few things, which you may not know the answers too and may not wish to say, which is fine.
How long has your step children mum (StepCM) been leaving then at home and going out drinking for? Years or is it recent? My dd is 11 and ds (who joined us by adoption) is 5. I can't imagine leaving them while I went out drinking. And I can't imagine potentially what kind of an alcoholic state the mum would be in when she got back from the pub. These would all be issues for me. I'm not against drinking, and a few drinks with telly or dinner is lovely but might a night in the pub may lead to her being drunk, in sole charge of the kids?
Is there a man in her life now and does he live with her. Have you talked to the girls about him, just generally, and seen their reaction. Don't say if you do not want to.
I don't think you would need to move 100s of miles if you managed to get custody, you may need to adjust your work pattern in the short term and seek some financial help with a child minder after school. You might even be eligible for respite care for the girls if you had custody (might you?).
I'm sorry to say this but if you really think their mum is neglecting them would they be better off in foster care. Are you sure they would not be eligible for this. Might some sort of short term foster care give the StepCM the wake up call she needs? I would imagine she and you would not lose touch with them if they went into temporary care as I would imagine they would not be eligible for adoption at their ages.
Can I ask, feel free to say no.... have you been to their house, what is it like? Is it dirty or clean (I mean really dirty, not just a bit of dust or clutter - because mine is petty cluttered and dusty at times!).
At the moment it sounds like the StepCM holds all the cards, she can say whether you see the kids etc, if the kids were in foster care might all that change?
I know you do not want to rock the boat but you can report this situation anonymously. If it were me, and I were planning to do it, I would not tell a soul, not my dh, not my colleagues, I would just do it, anonymously, given the kind of information that any casual neighbour or friend might know. I would not leave my name with NSPCC or social services, then when the shit hits the fan, as it may well do I would deny it. That sounds super sneaky and cowardly, and a big secret to carry, I know, and I am normally the least sneaky person in the world! But actually for those girls, I would do it.