Hello!
I'm going to try and help you out here, but I'm not nearly the expert everyone else here is so if I'm wrong hopefully someone will correct me. I'm currently doing Universal on a Sunday and Disneyland on a Monday. I was told it was best this was since Disneyland can get very packed due to the locals not working. If you choose to do Universal on Sunday just try and change Disneyland to Monday, Wednesday or Friday. Those days DL does NOT have Extra Magic Hours in the morning (DCA does) and you will be on equal footing with all other guests. Visit DCA on a Saturday, Tuesday or Thursday. Those days DCA does NOT have Extra Magic Hours in the morning (DL does) and you will be on equal footing with all other guests.
As for the front of the line pass. What I have gathered is that its very rarely needed especially if you get there at park opening. However, the good thing about it is that you can get it while you are there, so you can gauge the park before buying it. However, if its just you and your sister I would think if you get there at park opening you should be fine.