If you plan on BF'ing then I'd say attend at least that class as BF can be a struggle to start so any tips/advice and so on will be useful.
As for the others, I attended the labour/pain relief class and skipped the rest.
The thing to consider is although your sister in law had a baby 2 years ago and obviously is experienced, no two babies are the same. So what worked for your SIL may not work for you, so having other ideas to float around and discuss might be a good thing for you. There are many ways to tackle problems and so on, so only having your SIL might narrow your viewpoint.
For example, I am BF'ing but could not give anyone advice on how to get their baby to latch, suck, find the nipple, deal with sore or cracked nipples etc as my baby never had any problem whatsoever with anything to do with BF'ing and its been a breeze for me (other than he is very demanding for my boob
). However many women do find the early days hellish and a struggle (and that is often where/why some go over to forumula). So for many women having as much info about all aspects of BF'ing would be good IMHO and worthwhile.
Also then things like winding, posseting and all those things connected with feeding be it formula or breast fed.
Same with labour/pain relief class. Your SIL's experience of labour will of course be different from yours. So long as you are informed about all the pain relief optiions, breathing techniques (good for first stage labour) and the stages of labour etc then fair enough. If not it might be good to attend and hear others ask Q's and ask some yuorself if you have any. Answers from professionals so you know you are being informed correctly.