Hi Resham,
It seems you're concerned about your sister's "fickle" emotions regarding this guy. One minute she's planning her engagement......and the next minute she's flippantly saying that he can be "replaced." 21 is not too young for a girl to get married in desi culture. Around this time (21-22) most girls have either graduated or are close to graduating college. And there are many girls who have gotten engaged/married at this age and are mashaAllah leading happy marriages.
I think if your sister displayed more mature and serious behavior about this rishta.........you would feel more relaxed. But the fact that she herself can be so wishy washy about whether she wants to marry this guy........is the reason why you have qualms about their relationship. And that's understandable.
So, here is what I would suggest that you do. You need to talk to your sister about her "he can be replaced" **response. Start of the discussion by telling your sister that you love her and that you're happy for her and want what is best for her. **IF YOU START THE DISCUSSION OF IN SUCH A POSITIVE MANNER............your sister is less likely to accuse you of being jealous. After telling her that you love her/are happy for her/want her to be happy........PROCEED to discuss your concerns. Tell her, "I want you to be happy in your marriage because it's a life long commitment and not something you can end on whim. And although I'm happy for you, I'm also a bit worried because I'm confused by what you meant when you said that he's replaceable. I feel that if you're not sure about whether you want to spend the rest of your life with him......that maybe you should take some more time to get to know him before deciding to get engaged. I just want you to be sure of your feelings." And then listen to your sister's explanation about why she feels he's "replaceable". Tell her that while it's good for a guy to be educated, rich, and good looking..........it's also important for his personality to be compatible with hers. And tell her to make a list of pros and cons about the guy before reaching a decision.
IF you have already had this discussion with your sister and if it has not made any difference upon her, then you should talk to your parents about this privately. Tell your parents that you love your sister and are happy for her.........but the only reason you have doubts is because of her "replaceable" comment which indicates that she may not be serious about him. Tell your parents that money and education will get old fast if the personalities of the guy and girl are not compatible. And then ask your parents to at least talk to your sister to make sure if she is indeed serious enough about this guy to marry him.
As far as personalities are concerned, you have to keep in mind that many people have an introverted and reserved personality. This does not mean that there is something wrong with them or that there is any reason for suspicion. It's just that some people are less talkative than others. Also, keep in mind that people can sense if others are uncomfortable around them. Perhaps this guy has sensed that you're not too crazy about him and this might cause him to become nervous and more reserved around you. If you feel uneasy at the idea that you don't know him..................then GET TO KNOW HIM! Hang out more often with your sister and him and try to INITIATE conversations with him. Shy people are often times not good at starting conversations, they need others to get the ball rolling for them. So.............start discussions with him. Ask him questions. Ask him OPEN-ENDED questions that will prompt him to give you more than just one-word answers. If you ask questions that are only call for a one-word answer......the conversation doesn't flow much. So, ask open-ended questions. This will help you get to know him as an individual.
As suggested above........investigations are ALWAYS done during the rishta process. So.....do an** INTERNET SEARCH** on this guy to make sure he's decent. And get your parents to ask OTHER FAMILIES what they know of his character. This will make you feel more secure as well.
Also, I think you said that you're surprised that your sister can find "love" so young. Well....it can happen.......and it does happen. I've known girls who got married at ages YOUNGER than your sister and are very happily settled. I've known girls who are MORE FICKLE than your sister and they got married early and are doing fine. Encourage your sister to pray Istikhara. And also pray and make dua for your sister's happiness. In the end.......Allah will do whatever is best and whatever He has destined for us.
IF.......you are subconsciously feeling hurt that your sister was able to find "love" at a young age..............then realize that everyone's destiny is different. Don't let that get you down. Take pride in the fact that although you are older than your sister......that you are SECURE and CONFIDENT enough to settle down when the time is right for YOU as an individual...and that you have no desire to rush getting married just for the sake of getting married and fulfilling a societal expectation. Take pride in the fact that you are not going to give into STUPID desi society rules which dictate that the older sister must get married first. Such rules have no place in Islam and are devoid of logic. You sound like a confident and caring older sister........and when the time is right for you....you too will find an amazing guy!