stealing pregnancy thunder..?

my SIL is curently 23 weeks pregnant with baby #1 after 2 MC and I just found out I am 5weeks pregnant with baby #2. I was planning to announce on Christmas morning to my in laws and do a cute thing. get her a onesie that says big cousin on it and have my son wear a shirt that says big brother under a sweater and reveal it after she opens her gift. now I'm a little nervous she will be sad that I'm stealing her "pregnancy thunder" I mean the baby's will be 5 months apart so I didn't purposely try and get pregant RIGHT after her. should I go through with my Christmas announcement or should I tell her first in private so she knows I had her in my mind and didn't want to step on her toes..we are supossed to go shopping tomorrow to look at baby clothes for her daughter so it would be a good time. I'm just unsure of what to do.