You're ok. Cancerphobia is a real thing and can be very, very difficult to combat. I think the fact that there is a family history makes it reasonable that you are being diligent, but don't let it consume you.
Couple of things:
1) You had a sinus infection that took 2 course of antibiotics to fight off. That's a lot of infection to clear and those nodes in your neck is where alot of the fighting took place. They swelled to help fight things off and it will take time for them to shrink back down, sometimes a couple of months. Also, lymph nodes don't like to be touched so the more you poke it, the more inflamed they become.
2) you've seen your doctor who checked them and felt they were fine. Plus you have no other nodes to speak of. That's also reassuring.
3) There is nothing wrong with getting a second opinion if it will put your mind at ease. But, if the 2nd opinion is the same as the first, then you need to trust them.
Long story short - I've had an "enlarged" node in my neck for 3 years. My doctors could barely feel it, but it felt huge to me. Normal blood work, normal CTs, normal MRI, THREE ENT docs all saying it's fine (hey, I'm thorough), still obsessed endlessly about it. In the end what finally helped it stop hurting and slowly return to normal was realizing I was doing the damage to it by constantly prodding it and letting go. It's not easy to do and there are still days I have a small nagging voice in the back of my head, but it's been the best thing for my health.
When the labwork is normal, make a promise to yourself to leave it alone for at least 1-2 months. It's going to be very hard to do, but it's important that you give it time to recover. After the 2 months if you still think about it and it's not any smaller, talk to your doctor again. Most likely, though, after 2 months of leaving it alone it will be smaller or even back to normal size.
You'll be ok.