Hello Staranise- aka fellow tearaphobic- ddeeeeep breath and huge hugs to you.
given your baby is not due til december i think you actually have enough time to get yourself in the right place psychologically to deal with this. ask your mw to be referred to a counsellor asap (most maternity units do have counsellors attached) to work through your previous births and any fears/issues arising from them, in particular the tearing fear. the mws i have seen during this pregnancy were very sympathetic (admittedly i am going private this time...) and told me it's a very common fear and a very common thing that they encounter. they were all quite open about what they can and cannot do to help prevent tearing, here's what i can report back:
perinaeum massage- although most mws are quite evangelical about it, there is actually no firm evidence that it does work. i was very squeamish about it and one mw said to me "if it makes you feel worse, rather than better, DON"T do it!". if you do decide to do it (not until much later in pg) then i think wheatgerm oil is supposed to be the best as it's got lots of vitamin e which softens scar tissue
positioning- certain positions (all fours) are really meant to help- so discuss this as a possibility
water- is supposed to help, but no firm conclusion
supporting the perinaeum- mws can do that during second stage, again you may want to ask
re pain relief- ultimately it is YOUR choice, if you really want it again discuss it calmly and openly and make sure they understand it's not just a whim but a real phobia
if you are seeing a consultant (i did at about 16 weeks) ask him/her to really look at your scar/s and tell you his/her opinion of the state of your perinaeum. mine did have a good look and poke around and told me that she was not concerned- all was as should be and that reassured me enormously. much better imo than having a theoretical conversation about elective c/s etc.
but having "conversed" on this topic for a while, i really do think the most important thing is to work on the psychological side of things and put previous births behind you. AND remember- what happened to your sister doesn't necessarily have to happen to you!!!
sending you huge hugs, take good care (and i really, really, really do hope that in a few weeks time i will be able to report on a positive birth experience- in truth i am shi**ing myself with fear of tearing!!!)
oh and... it may be a good idea to stop googling "tearing in childbirth"... come on you know you are guilty