What your friend has 'hinted at' might actually have at least some substance to it.
Since your sister isn't paying a commercial rent, a court might regard her as being (in part) 'maintained' by you. The Inheritance (Provision for Family and Dependants) Act 1975 gives "any person . . . who immediately before the death of the deceased was being maintained, either wholly or partly, by the deceased" the right to apply to a court "for an order . . . on the ground that the disposition of the deceased’s estate effected by his will . . . is not such as to make reasonable financial provision for the applicant".
So, if your sister thought that your will ought to have made provision for her (or greater provision than was actually made), she could ask a court to vary the terms of your will so that she received 'reasonable financial provision'. The court wouldn't
automatically make such an order but it would consider the matters referred to in Section 3 of the Act and make a ruling based upon them.
http://www.legislation.gov.uk/ukpga/1975/63