I'll start with a brief* word of advice. (*it grew :-) )
If you value the friend's friendship and that of her sister and new husband, then the chances of them being damaged as a result of your endeavours are high enough to not be worthwhile. You MAY turn out to be a born natural wedding photographer and produce astoundingly marvellous results despite your complete lack of experience, equipment limitations and what all too often happens in such situations. BUT what happens all too often is .... do I need to spell it out.
I describe myself when people ask, to provide a label they may understand, as a "semi professional photographer". I'm an EE "by day" but photography is an obsession. It more than pays for my equipment if you count my time at say $1 per hour (or less)(post event editing et takes far more time than the actual photo taking). As a hobby that's great. Most of my photo events are unpaid for. I have photographed stage shows, dancers, 21sts, 1sts (a big thing with some), funerals, receptions, festivals, ...., and weddings.
I have photographed maybe 50 weddings total. Maybe more maybe less. People like my results, they say. I do not advertise and get a slow trickle of requests. AND wedding photography is, in my opinion, the hardest, scariest, most demanding, most real time, most unforgiving type of photography I've met.
A lot of it is standard enough. But 1st kiss, giving of rings, giving away the bride, walk up the aisle with the quick look at a friend or a glance between father and daughter, and similar only happen once. No second chances. No "wouldn't focus", exposure wrong, person in the way, battery flat, memory full, camera "just died" (and yes, I've had that happen), lapse of attention, ... matter afterwards when the special day photos turn out less than ideal or complete. Bad exposure, out of focus, too far away, nasty lighting, bad colour, poor framing, bad background, ... , matter at all afterwards.
I photographed a for-$ 21st birthday a few weeks ago. I had the camera set to "focus priority" rather than "take-the-photo-now-the-focus-is-good-enough-just-do-it" mode. I never usually do this. When the young lady kissed her mother after being given her 21st "key" the camera decided not to focus in time. Very unusual indeed. Very embarrassing. I asked them to do it again and I got 3 good shots. If that had been a wedding and first kiss it would probably have been a permanent missed photo. [And 2 weddings back I had a bride and groom who decided for what ever reason to lean in for one super-super-super fast kiss and that was it. I was ready. If I'd missed it the fact that it was "their fault" would not make up for the missing "essential shot. How well does your system catch that sort of thing. Does it matter to you? Will it matter to them? ]
When you have the guts, nerve, stupidity, gear, experience and more to risk spoiling somebody's special day, give it a go. Until then ... .
I love photographing weddings. I enjoy almost every one immensely. At the end of the day I'm sore (almost agony)(spinal fusion this year helped :-), exhausted - and usually very happy. And almost every time I tell myself that it was too scary at the start and I'll not do any more. Until next time:-).
BUT If you insist:
I do not mean to be rude about the equipment - in many cases it will allow you to get excellent photos and its good enough for many purposes. Assessments below such as "marginal" or "poor" are in a wedding context. Don't feel bad about the lenses for most purposes. (In more relaxed circumstances your 18-55mm lens can do things like this )
Your equipment is ultra marginal in this context.
Lenses are presumably 'kit'.
"Real time", possibly low-light, demanding focusing, get one shot, fast flash cycling, ... is much much easier and safer with 'good gear'.
You do not mention external flash.
On camera flash is better than none at all, but lacks power when power is needed, cannot "bounce" so must direct illuminate with more risk of harsh and uneven light. On camera flash uses camera-battery energy so you get less shots and need more batteries. You have to take more care not to run out of battery power at critical moments. Flash cycle times get worse with declining state of charge and you have to remember to allow for this so as not to be caught out at crucial moments**.
The 75-300 will be too hard to use for various reasons most of the time.
Presumably the 18-55 is a kit lens.
Quality will be marginal or worse.
Low light performance will be poor.
The master would find it challenging to do a good job with it. You'll find it much harder.
Batteries and memory cards matter and must not be limiting factors.
Practice as much as possible in realistic circumstances.
You MUST have some sort of equipment backup - plan B if gear fails is ESSENTIAL.
The camera could produce superb results in the hands of a master. Not being a master would make it more challenging.
Thinking of charging at this stage is a bad idea.
If it's not fun enough at this point to do it for free, definitely do not do it.
IF you can get several people to work together then you may be luckier with results. Or not.
The urge to do your friend a favour is fine - but the better favour may be to not do it. If genuinely nobody else with more experience and better gear is available and affordable and it's you or the iphones :-( then you may have no choice. But hopefully there is a better way.
Ideally - Dont!.
For flashes that use AA batteries, I use NimH rechargeables when time to change is not utterly crucial. For the key portions of a wedding or eg stage shows where things happen extremely fast and you want to always be ready, I use good quality AA Alkaline batteries. These have the advantage that they can be discarded or dealt with in a minimalist manner. A set of rechargeable batteries that has value and has to be put somewhere safe so they are not lost but also do not get mixed with new batteries can take precious seconds when it really matters. Under very heavy use batteries (either NimH or Alkaline) will come out of the flash so hot they they cannot be handled (somewhere over 60 degrees C!). Not needing to put them somewhere about your person is most welcome.
Raw & JPG: If your camera will shoot Raw & JPG and you never usually use Raw, try to use both. This takes more memory capacity and may slow down per photo write times. You may not use the Raw images most of the time. But, if a crucial shot has exposure or colour balance issues, a Raw image will increase your chances of image recovery.