Congratulations on the offer, Rae! While vacation and benefits are indeed a positive, honestly if I were in your shoes I would consider a counteroffer.
Many people are afraid to counteroffer because they don't want to seem greedy, difficult, or risk losing the job. There are many reasons to counteroffer, but the fact your co-worker, a male, estimated the salary as double makes me think they are low-balling you. Don't feel bad - this is very common with both younger workers and women.
Most job offers allow room for a counteroffer and some hiring managers/HR even expect a counteroffer. Even if the current offer is the best they can do, they will let you know by standing by their offer. It is very, very rare for a company to renege on an offer after they receive a counteroffer. If they would renege though, that is definitely not a company you'd want to deal with and you'd be lucky to know that upfront.
So I would make a reasonable counteroffer and see what they come back with. Remember, you can negotiate on elements in the offer aside from salary, including flexible hours.
