If your sister really likes that man and really wants to marry him, then you should first make it clear to her that there is a problem in marrying a distant place, and make it clear to her that if she is wronged, she can't let her family help her. And going there may be a stranger. What's more, your family is also worried about her, so maybe your parents will be worried about her after she gets married. If it is really a long marriage, then you must not regret what you have wronged, because it is her decision now. But in the end, you should tell her something she should pay attention to. After all, your sister went to her mother-in-law's house after marriage, and there must be a lot to pay attention to in her mother-in-law's house. Then I think you and your parents can tell her more things that should be paid attention to in your mother-in-law's house.
I think anyway, after all, your sister made a good decision, so I think I'd better give her my blessing. Even if you are not at ease, you can always call her and ask her if she has been wronged. If she is really wronged, it is not too late for you to help her. I think she would prefer you to give her your blessing at this time.