HELP!!! (hope this doesnt get too long!!)

I wanted to throw a baby shower for my husband's niece - this is their first and she due in late August. So DH's sister texts me last week saying she wants to discuss the "when" as niece's in-laws will be in town Memorial Day weekend (i wanted to have it at the end of June). I told her we can discuss after Easter Dinner - which we did. I had asked niece to give me a list of people to invite - next thing i see is that she has basically planned out the entire shower including where they are going to have it (at her house) and most of the food. They are getting tables and tents from the church.

I jokingly told the father that all he had to do was come to my house when it was all over and take the "loot" home. He said "OH NO IM NOT - its going to be at our house". I was stunned.

I dont know what to do. Should i just step back and let them handle it? Should i stand my ground and somehow tell them that i was going to have it either at a restaurant or my house (or at my community center) and that i was going to foot the bill for the food. I had definite VISIONS for the baby shower and NONE of them will happen if they take over!

I can deal with the fact that they want the in-laws and i dont mind having on a holiday weekend but just to take over the entire planning is was really irks me!