Either and or are called correlative conjunctions when they appear in tandem. They present several grammatical and usage issues, and because they’re used so often, you’re well advised to commit a few rules to memory.
Use them with paired items only. Once you get to three, drop either.
Make sure the items they frame are grammatically parallel. You can say He either forgot it or ignored it. But you shouldn’t say He either forgot it or he ignored it. The grammatical element that follows either must match the one that follows or. (In the instance just cited, the he is extraneous.) Upon seeing either, any good editor immediately looks for or to ensure grammatical parallelism.
Remember that if these conjunctions frame alternative subjects in a sentence, the verb’s number is determined by the second element. Hence Either he or his sisters were to blame. Or Either his sisters or he was to blame. If such a construction sounds odd to you, just reword: It’s pretty clear who’s to blame: either he or his sisters.
I could burden you with minutiae, but your time is too valuable. If you forget these rules, from some readers you’ll either attract derision or evoke pity.