It should be a choice for each of you whether or not you sunbathe topless and those who choose not to should not be criticised. On holidays to Spain to what is probably a similar kind of resort there have been groups on a 'girly holiday' there have been some topless and others not, and no-one has really cared. I now go to France myself (friends have a flat I can use, and I speak enough French to avoid ever having to say 'do you speak English?) and there is a similar attitude akin to 'it's your choice'.
So if it's one of your group topless or whether it's six (you and the four friends who might) makes no odds. Your having sunbathed topless in front of family members suggests you should be comfortable doing so on this holiday, as I've come across young women who will sunbathe topless with friends or with their boyfriend, but never if on a family holiday.
Whatever you and your friends choose, don't forget your sunglasses and plenty of suncream. And a willingness to be firm and blunt with any man who gawps at you (as someone I know puts it 'talks to your tits not your face').