No it isn't. I guess it the wife of one of the Knights of the Garter as I don't recognise her as a family member but I am happy to be proved wrong. Princess Alexandra is a Lady Companion of the Order so wouldn't be standing waiting for the procession with Sophie and Catherine. The Duke of Kent was drive just ahead of the procession in his robes in a State limousine. I guess if Princess Alexandra was attending she would have joined her brother in robes and in a state limousine rather than walking in with the spouses of the Companions of the Garter.