Gigi Hadid has been keeping a low profile since giving birth to her first child in September, but made sure to return to social media on Friday to mark her sister Bella Hadid's birthday.MORE: Gigi Hadid's baby's name and first photo - all we know about model's daughterThe 25-year-old model shared a series of sweet throwback photos of the pair from over the years.These included a childhood picture of her cradling a newborn baby Bella, and another snapshot of the new mum and her baby bump posing