I was having a convo with 2 aunties yesterday, one born and raised in UK and the other born and raised in Pak...both religious. I came to know that Islamically, a guy and girl don't need their parents approval to get married. Islamically if 2 people like each other, they can get married, since marriage is farz and they aren't doing anything wrong. However in our made up cultural society's eyes, it is wrong. Just like how our culture doesn't accept nikkah as shaadi, and only allows guy/girl to live together after "ruksati" sigh. Your brother is old enough to marry whoever he wants, when he wants, if he wants. He knows whats best for him. Same with your sister, she is old enough to know right from wrong. Best you can do as a sister is advise her calmly if she does do something wrong. Socializing with guys, having guy friends, as long as it's in public, and they aren't alone, it's totally okay! :)