Oh no I'm in a jam! What should I do?

First I want to thank all my new friends for the much appreciated advice and comments you left me concerning my previous question. Since you were all so smart and sensible I have another one I want to ask. Okay here's the situation: My girlfriend just got pregnant. Okay she's not my girlfriend but she's my girlfriend's best friend. Her best friend happens to be her sister. However I don't want anyone to know what I've done so I've come up with a plan. I'm going to organize a hotel party and only invite my brother and my girlfriend's sister. I'm going to leave a bunch of alcohol in the room and not show up. Im going to tell my brother how cute my girlfriend's sister thinks he is than likewise to her. They're both kind of freaky so I don't have to do much more than this. Then I'll p it off as my brother being the one that got her pregnant. I guess my question is, is this morally wrong? Any input you give will be wonderful. Also if anyone has a better idea than this one since its kind of amateur I'd appreciate that too. Thanks.