Is this a normal relationship that my boyfirend has with his sister?

it is more normal than you think
most guys fantasize about mother and sister
most don't do anything about it be cause it is supposed to be taboo
what you feel about it get your feeling straight
talk with him and accusingly
find out from him where you fit into this sittuation
and live with it or move on
in genneral people need to be more open minded
as long as bro and sis are not hurting anyone then leave them alone