How to tell my fiance's mother we're pregnant.?

She pretty much hates me, even though she pretends like she doesnt so that she can complain about her ex, his father to me. I love his dad and stepmom and while they will be very disappointed because we are younger than they would prefer, they will be understanding and open to listening to what we have to say and will be supportive. I know that shes going to fly off the handle and be like youre too young and now youre stuck together forever and all this bs that i dont want to hear but i kinda wanted to tell her in a cute way like get a "i love my grandma" picture frame or something and put a sonogram pic in it.. but idk if thats a good idea if shes going to be a ***** about it.. i know she will but i also know after the initial shock she will be up our es to give advice and invite herself over to see the baby(s) ...(twins run in his family, i know im pregnant not sure with 1 or 2 l0l)..help!