Look if she is in pain then Islam allows the massaging of feet of a non-mahram. If u are not massaging for purpose of reliving her pain then its haram (because she is non- marham). Hazart nabi Muhammad (PBAHAHF) said its OK because u are providing medical help to someone. If it was your mother's sis then she be marham. Besides health issues it is haram to touch non- marham feet such as uncle wife.