i was sort of in the same situation with my mil
my partner had a son with his ex he is now 2
but wen we found out i was preg she went and bought a teething ring a booster seat and clothes in size 2.
To some extent unfortunately you have to grin and bear it but maybe try letting her know that your feeling a bit left out and see if she can include you in and ask you for your feelings on things she wants to buy and then you knw what she has bought so if anyone asks what you need you wont get 2 or 3 of things.

A tip for all the teddys i got a a mosqito net that goes over the bed and made it into a hammock on the ceiling but there are also toy hammock and you can place the teddy's in there then they are ou of the way but they are stll in the room to make her happy.

Unfortunately i have a new mil problem lol
she was in the labour room with my partners ex when she was giving birth (to much of his ex's disgust) so now she is demanding she is going to be in the birthing room with me but i refuse so now im the biggest ***** in the world lol.

Hope you have some luck with your mil