I'd go for the pile of poo gift card.
Or don't bother with the gift card and just send her a pile of poo....
This reminds me of the very similar recent thread with the OP whose brother demands precise, expensive gifts for himself from her for every birthday and Christmas but never even gives her a card. For reference, the brother has autism, but as a lot of people pointed out, it's the fact that he has selfish-arseholery that actually causes this behaviour and not the autism.
I don't want to be harsh, but if she would pocket cash and spend it on booze, she could just as easily steal the gift vouchers, use them for her own regular shopping and then spend the shopping money she's saved on booze. Otherwise, she could just sell the vouchers. I don't know her personally, but I do know there are people out there who would be mentally calculating that they could flog a £50 voucher for £35-£40, so that would be its 'booze value' to them - like when drug addicts will often steal to pay for their habit: they couldn't care less that you paid £800 for the laptop, because it can get them £50 down the pub. Absolutely anything can be sold by an addict, and there will invariably be a very high percentage of 'wastage' in the value between that paid by the owner/giver and the amount realised from selling it.
By the sounds of it, pretty much anything you give is liable to be stolen/re-appropriated by her to free up money for booze. If the girls do get the presents from you, are you certain that she tells them that they're gifts with love to them from kind Auntie & Uncle, or might she pretend they're from her and then not buy anything for them herself?
To be frank, if you're currently needing to rely on charity for your own basic expenses (no condemnation whatsoever), you are not in a position to be spending a lot on Christmas presents. I would have thought that a £5 gift each would be more than reasonable under the circumstances, or even a Poundland selection box if you can't run to £5.
If whatever you give is likely to be 'translated' to booze one way or another, stop and think whether you want to show your love for your nieces by buying their mum £5-worth of booze or £100-worth of booze whilst they get nothing either way. That might help to put it more into context.