Heres some info about how twins are developing at 28 weeks...so amazing...
Your twins' brain growth is very rapid this month. The characteristic grooves on the brain's surface start to appear and more brain tissue develops. Nearly all fetuses react to sound by the end of the seventh month.
At about this time, your twins can open their eyes and turn their heads in utero if they notice a continuous, bright light shining from the outside. Their fat layers are forming. In boys, testicles descend from near the kidneys through the groin en route to the scrotum. In girls, the clitoris is relatively prominent because it's not yet covered by the still-small labia. These will grow to cover it in the last few weeks before birth.
Their skeletons are hardening even more. While your twins may soon slow down growing in length, they will continue to gain weight until they are born.
A pint and a half/around a litre of amniotic fluid now surrounds each of your twins, but that volume decreases as they get bigger and have less room in your uterus.