DearSarah，
IamterriblysorrytotellyouthatIamunabletoattendyourbirthdaypartynextThursdayevening。Thatisowingto(因为)thefactthat(同位语从句)myyoungerbrothersuddenlyfellillandwastakentoahospitalthismorning。Ihavetogothereimmediatelyandtakecareofhim。Astoldbythedoctorincharge，itwilltakearoundfivedaysforhimtorecoverandIhaveaskedmybossforaleave。
IreallyregretthatIcannotgotocelebrateyourbirthdaypersonallyandwouldmisstheperfectchanceofenjoyingmyselfwithallouroldfriends。Ihavechosenasmallgiftforyouandwillsendittoyoutomorrowtoshowmybestwishes。Besides，pleasegivemyregardstoourfriendswhenyoumeetthemattheparty。
Cordially，(sincerely)
LiHua