I need your help.

So long story short my husbands step mom said she was planning my shower. Which okay cool. Well she hasnt talked to me about ANYTHING that we want for the shower. And I'm due late August. She wants me to have my shower in July but I want my shower in mid June because I want all of July to enjoy my pregnancy and if I dont get things off my registry I want to have time to get them. Well, she said itll have to be the last week of June because shes basically gonna be gone the first week of may until the first weekend of June. That doesnt give us any planning time nor does it give my guests enough advance time to plan. My sister in law and aunt offered to take over so I dont have to stress and I can have it when I want and how I would like. But I dont know how to tell his step mom that they're wanting to take over with out me sounding like rude, or a bitch. Plus, she can be very very petty.