Peter Phillips’ daughters, Isla and Savannah, appeared to be getting along famously with their future stepsister, Georgina Sperling, on Easter Sunday. Georgina, 13, made her royal debut at the service held at St George’s Chapel, Windsor Castle, alongside her mother, Harriet Sperling, who is set to marry Peter this June.  © WireImageGeorgina Sperling (centre) was supported by her soon-to-be stepdaughters Isla Phillips (left) and Savannah Phillips (right) Welcoming the teen into the fold with open arms were her future stepsisters, Isla, 14, and Savannah, 15. The daughters of Peter and his first wife, Autumn Phillips, walked side-by-side with the newest member of their family. Isla looked strikingly grown up just days after her 14th birthday, smartly clad in a polka-dot dress, navy blazer, and suede loafers. Taking the lead as the trio walked together, eldest daughter Savannah impressed in a structured coat dress with contrasting sleeves and white kitten heels.  Georgina opted for a navy button-down top and brown suede midi skirt for her high-profile debut. Their coordinating ensembles signalled a shared confidence, suggesting that …