Need advice

Well after 3 months of personally knowing my 41 y/o mother was impregnated by her 20 y/o bf 
She finally tells me today after I saw her stomach getting outta the car 
And I'm really pissed about it 
And tomorrow after school she want me to go to hobby lobby to get shit 
To announce it on fb or what ever 
But I told her no "I didn't wanna go"
And she wants me to be apart of this whole "pregnancy" adventure 
Like go to appointments and stuff 
But I don't really care to go and not excited what so ever 
(Not an only child have 4 other siblings one being younger and 2 having different dads) 
I didn't know how to handle this situation because I don't wanna be apart of it like at all...