I am no expert on this but I think the boss is about to break the law. Is your sister a union member? If not, she should probably go to the CAB. SMall companies (with fewer than 10 employees or something) are allowed a bit more leeway as it is harder for them to manage maternity leave/part time work etc - but a large retail chain is not exempt from the law.
boss cannot sack her, deny her her legal right to maternity leave, or treat her unfavourably because of her pregnancy. Have a look on the Equalities Commission website and ACAS. If boss does any of those things, your sister can go to an industrial tribunal and sue for sex discrimination.
As it's a big company, would be worth involving the HR dept. who should be much more clued up.
What is the company's maternity policy? How much is paid?
She needs to get in touch with the H/R department before she does anything else. Her boss may actually be right, albeit put across very crass; that she may not get 5 months paid maternity leave, and that there aren't any part time jobs available.
But urge her to get in touch with H/R before she goes to any external agencies. They will explain it better, hopefully.
Might be worth reposting this in Employment issues.
It all seems frightfully illegal but Flowerybeanbag is your best bet to get all the correct info.
My hazy knowledge is that all woman are entitled to return to their old jobs provided they take less than 6 months Mat leave, if you take between 6 - 12 months then you are entitled to return to one at the same grade but it may not be the job you were doing before.
Regarding part time working you are entitled to request it but your employer can refuse it based on a number of business reasons.