I'm afraid that what you wish for is only achievable by surgery. Whether you can have this done in India or not, I don't know. I'm told that there are some outstandingly good cosmetic surgeons in India, but as far as I'm aware, penis enlargement is available in America, the UK, and some European countries, and Brazil. Brazil is fast becoming the centre of the universe as far as cosmetic surgery is concerned.
There are no pills, potions, lotions, techniques, or apparatus for making your penis either longer or fatter. Everything you may have seen in magazines and newspapers is nonsense. If someone could genuinely invent something that would make a man's penis bigger, don't you think it would be on the 6 o'clock news as soon as some such was discovered??? These adverts you see on the Internet, the papers, magazines et cetera, are all designed to make gullible men part with the maximum amount of money, and the only thing that gets larger is a bank balance of the unscrupulous swine that make these adverts up.
So, ignoring all the bogus nonsense, you are left with surgery if you really want to enlarge your penis. This absolutely has to be done by the most reputable surgeons around. There are plenty of examples of these things going wrong in the hands of the incompetent. When surgery like this goes wrong, it often results in a penis that is functional only for urination, as the loss of sensation, and the scar tissue, render it unfit for sexual purposes. Nor can you expect a really huge increase in size. From the information that I have read, you may gain a couple of inches, and a certain amount of thickening. The thickening is achieved by taking fat cells from other parts of your body, in order to increase the girth of your penis. The increase in length appears to be largely due to cutting and re-stitching a ligament that allows more of the penis to protrude from the body. Effectively lengthening the shaft, though in actual fact, all you are doing is taking part of the shaft is normally retained within the body, and moving outward. There are one or two other things that can be done, but these actually require surgical implants, which are actually intended for people who suffer from erectile dysfunction that does not respond to the popular drugs used to that problem.
There may be new surgical techniques that have been developed recently, but as a woman, it's not something that I keep up-to-date with. You really have to do your own research, and I would suggest that you go online and do some investigation. Be wary of websites that are trying to sell you something. Any website that finishes in .org is less likely to be promoting something, and is more likely to be giving genuine information. Please do not be taken in by the of these adverts for pills and potions or vacuum devices et cetera. Look at websites offering cosmetic surgery, and then read about the procedures that they use. Bear in mind that the cost of surgery is high, and also bear in mind that any surgery does have a risk attached to it. There is a very small but significant chance of brain damage or death during any surgical procedure that requires a patient to be unconscious.
Now let's consider other things that you can do with your wife's sister in order to satisfy her. I'm not going to question the morality of what the two of you are doing, that's your business not mine. You should think of possible consequences if you get caught, but again what you do is up to you.
Satisfying a woman is not that difficult if you are imaginative in the things you do with her. If she enjoys strong physical penetration, then I would suggest you buy an outsized dildo, or if these aren't available, buy a suitably shaped vegetable such as a cucumber, a large mooli, or a large corn on the cob (or whatever suitably sized vegetable appeals to you). You can push these into her vagina or anus, whichever she prefers, or even both if she likes double penetration. Using your lips and tongue on her clitoris while you are pushing a large object in and out of her, with as much force as she desires, should allow her to achieve breathtaking orgasms. Her clitoris is the main point for stimulation, and to be honest, for most women, effective stimulation of that little button is all that she really needs to give her the most euphoric sensations of pleasure.
When it comes to penis size, it is more a case of psychological excitement than anything else. It is often the case that a woman who sees a lot of pornography, sees these studs with huge cocks, and starts believing that all guys are supposed to be built that way. What's even worse about this is that guys such as yourself finish up having an inferiority complex because they cannot hope to match up to the freakishly large penises that they see in porn. Try to remember that these guys are not selected for their Oscar-winning acting abilities, they are merely there because they are freakishly well endowed. That is their only claim to fame, though some of them do have extremely good bodies as well, as the result of working out, and probably steroid abuse as well. You are just an average guy, and there's probably nothing wrong with the size of your penis. You are just being compared unfavourably with some porno actor. If you get your sexual techniques really well honed, then the size of your penis will be incidental. Make a point of finding out what she really enjoys, and don't be scared to experiment in techniques and activities that she may have never thought of before. Ask her everything about her sexuality, and don't be reluctant to try pretty much anything. The more erotic you can make her feel, the less she will be concerned about penis size, as she'll be having too much fun to even be thinking about that!!! Throw off any inhibitions you have, and get her to do the same, I'm sure that you will find that the two of you have very thrilling and enjoyable times together.