Is this a word made of other words?
Yes. It comes from the verb tuismigh and the suffix to create an agent noun -óir. Literally means "Procreateor"
How do you get the longer words for brother and sister. When do you use the long terms versus the short terms?
How's that pronounced (wihout lenition)? Like "tuismióir"?
Only the first h is inserted during lenition, so the base word is tuismitheoir. Basically take this phrase's pronunciation and say "tish" instead of "hish".