Is it normal for a man to be very protective and overly close with his sister?

my boyfriend and his sister are very close and he is way too protective and acts like her boyfriend, every time he hears she is out he wants to go there immediately, and wants to see who she is with and does not let her live her life, his sister is also very dependant on him and always wants to be with us, and when we together they would just talk to each other and i would be left out, they even talk openly about sex and stuff, i would just like to know if this is normal? she is 20 and he is 23
Is it normal for a man to be very protective and overly close with his sister?
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