Exactly, people will always try and tell you what they think about this and what they think about that but just take it one ear and out the other. I am a young bride, at 21 and still in school so usually when people tell me not to marry, its not in a serious manner but they are usually just joking around. People will always say something and sometimes they don’t realize it is effecting you and sometimes they can even mean to say on purpose but as long as you are sure of yourself, and you are confident in your spouse, then the two of you will be just fine. 
Fights and arguments can happen along the way and make you second guess yourself but thats just come with marriage, and well any kind of relationship really. If your fiance is willing to compromise with you on whatever differences you may have then know that you have one of the good ones!
I know its hard especially when family or sister like you say are constantly filling your head with these kinds of doubts, could be maybe because you only talk to her about all the bad things in your relationship. If you and your fiance argue then don’t tell anyone about it because it changes their opinion and respect for him in ways you won’t even realize till later on. If he truly makes you happy, then keep it inside no matter what is is, and if you know that going off of her advice or anyone else for that matter may cause tension in your relationship…then the only person you need to listen to is yourself. And please trust me when I say that, you may not realize and that by venting to your sister about something that happened between you and your fiance is only worsening the situation. It is not helping you or him. I emphasize that because I have seen what happened with my brother in law. My cousin and her husband had a very rough first year and a half in their marriage, and the only people she would turn to for advice were her close friends and family. Not realizing that they were growing to dislike him more and more because all they ever heard were the bad things about him. She eventually realized she shouldn’t of never said anything to anyone because eventually people started to give her and her family tanas for marrying him and her friends were all talking about divorces and convincing her that it wasn’t going to work and she would get really upset at that because she may have just been venting her problems since she was alone in a new city with a new family…she was never even thinking as far as divorce or anything like that. She just needed to talk to people about it but that backfired on her. So khair…her and hubby are doing great now mA, going on three years and having a baby.
Moral of the story…there is no doubt that marriage isn’t easy. Even for those in love marriages but if you are only unsure because of what OTHER people are telling you…then you need to really think hard about whether or not you will be able to live with this person for the rest of your life because only YOU know what he is like as partner and husband. Right or wrong for you, only you can decide that for yourself.