BM question

My sister in law is a bridesmaid and she is also doing a reading. She just had a baby last night and during the pregnancy she gained much more weight than anybody had anticipated. My mom and I don't think that the dress will fit at all, even though we have 4 months until the wedding and even if she loses a lot of weight. Now, should I wait until she tries on the dress mid Sept and see if they can order extra fabric than or should my mom say something sooner? She is definitely not gonna like someone saying that to her... I have another friend that I should have put in the wedding, rather than my maid of honor, so if the dress doesn't fit, I can always ask my friend who I want in the wedding anyway, which I would like to. My sister in law takes everything I say the wrong way and I can never say anything right to her, so my mom is gonna have to mention it, but I have a feeling this is not gonna go over well

is there any way you can order the extra fabric without her finding out? this way when the gown comes in and doesnt fit, they the seamstress can just say ' oh lucky for you, we have extra fabric'

I could order her some extra fabric, but to be honest, I wouldn't mind if my friend took her place I'm so going to hell