Your SIL is an idiot, or a cow.
My first birth was my best (of 3), 14 hours from first twinge to baby out, painful at the end but manageable with gas and air, not one tear! My mother's first birth on the other hand was terrible and she and my sister nearly died, so lucky things can happen!
There's nothing you can do to guarantee a perfect outcome, but there are things you can do to help, and the first is to calm yourself, though I know this is easier said than done! Have you done any hyonbobirthibg or even regular mindfulness or calm dvds? They are lovely for the early stages and help you feel less panicked or out of control (because once you start freaking out, it's hard to get back to calm). Your partner can help too by supporting your calm, whether that means massaging your back, telling you that you are strong and amazing (which you will be!), letting you break his fingers while gripping his hand during your contractions/c-section, or telling SIL to back the hell off!
Statistically you are likely to have some tearing (though plenty of women don't), but small tears really do heal fast, mine are not even visible now they've healed. My sister's c section scar got horribly infected and caused massive issues - c sections are amazing when needed, and i personally think women should always have a choice, but do be aware that on here people are much more likely to share horror labour stories (people with good births don't spend so.much time thinking about them!), while c sections can be presented as a risk free alternative, which is also not true.
Perineal massage feels a bit weird but has evidence that it reduces the chance of any damage, if you want to do it you'd need to start asap.
If you're really nervous do tell the midwives, they've been there before and can help.
On just three weeks or so, you will be holding a wonderful baby, good luck xoxo