When is it too close to a family wedding to get engaged?

My oldest son is getting married in three weeks. My youngest son just got engaged last weekend. He and his new fiance have no intention of publicly announcing their engagement at or prior to his older brother's wedding. She isn't even going to wear her diamond to the wedding to avoid questions. The only ones who know about it are my husband and myself, the brides parents (who live out of town) and my oldest son and his bride to be. We plan to keep it a secret until after the wedding. After my youngest son called his brother and fiance to tell them the happy news, (he had told them a couple weeks ago that he might be proposing soon) he received a nasty text message from his soon to be sister-in-law absolutely chastising him for being so selfish by not waiting until after their wedding to propose. My youngest son and his new fiance are sick about it. Is there anything wrong with proposing a few weeks before a family wedding as long as you don't make a public announcement until a couple weeks after the wedding??