So I am, but if so you have answered your own question. The vampires didn't need an invite.You're mixing up the Petrie's with the Glick's. One of the Glick boys was outside of Mark's window, but Mark grabbed the cross to ward him off. So, the question is still out there: How did Barlow gain access to their house?
Ooooh! Quite possibly!Yeah, they did need an invitation in every other instance. My thinking is this: Barlow crashed into the kitchen through the window and immediately smashed Mark's parent's heads together, killing them instantly. Since, as of right then, there was no rightful owner of the home, he was free to come and go as he pleased. Whatever pain or barrier to entering a home uninvited was apparently bearable for the 30 seconds or so (according to Callahan) that it took to kill them.