I'm having two issues, the first is when do you NOT put an "a" in front of athair, sheanair etc etc and when do you use leat/leibh?? There must be some rule for both as I seem to be picking the wrong times to use/not use the "a" and making the wrong choice for leat/leibh.
Yep, this exactly re: leat/leibh.
The vocative is used when directly addressing somebody e.g. 'Hello, sister!' You can't put an a in front of a noun beginning with a vowel in the vocative case purely because it's really hard to say. So you just drop it.
tidsear > a thidseir
màthair > a mhàthair
ollamh > ollaimh
athair > athair