What do I now?

This past weekend my two cats escaped. They have done this before and have always returned same night. This time only one returned while the other one did not return until two days later

Ever since then the cat that returned first is hissing growling and even snorting at the other?

That are brother and sister both fixed And just over 10 months old is there something I am missing or something to do to achieve peace again?

The first thing I would start with is to get them checked out by the vet, just to rule out the aggression being caused by a non-visible injury or illness.

Ruling that out, you may need to re-introduce them to each other. After being out, they likely smell a bit different and  some cats don't take long to "forget" their prior living arrangements.

You can find the steps for separating and introducing them here:
[article="32680"]How To Successfully Introduce Cats The Ultimate Guide  [/article]
You may not need to take too much time, as hopefully they'll remember one another and fall back into their original patterns, but this is probably a good place to start.