My sis in law sent out invitations for a house warming party to celebrate her and my brothers new home!  The party is set for 5 days before her 25th bday. I sneakily snagged up her guestlist and want to send out a note/letter/new invite to all her guests saying my bro and I are highjacking her house warming party and making it a SURPRISE bday party! This is gonna be very tricky since she'll be planning the house warming party but my thought to convert it to a surprise bday party is by getting a cake, balloons, and some festive plates and bringing them out after the first hour of the party and having everyone gather behind me as I carry out the cake and all start singing happy bday to her (my bro can help me hide em in the basement without her knowing for the first half of party)!!!!  What do you guys think?  Any tips or ideas for the invite phrasing or for the actual carrying out of the surprise party??  Thanks in advance!!

Zozzie:  thanks for your suggestion but I know she will be excited about this idea. She had talked about doing it as a dual house warming-birthday party previously but she felt silly planning her own birthday party.