Adult birthday party gifting demands

So I got roped into co-hosting a birthday party for my cousin. I'm fine helping but this party and the expectations have gotten really wild. I'm apparently in charge of all decorations and set up by myself. Well I'm a mom of two kids and an infant to set up this party isn't a small task. Also much more money then I was originally comfortable to spend.

I hosted a planning party after the mom of my cousin demanded seeing my party decorations. While here she wanted me to post a wish list and say to mail cards to her for those who cannot attend.

I nicely phrased if people want to send something they are adults will ask or know how to mail a card. That it isn't really acceptable to announce gifts and cards to an adults birthday. She asked why I have wish lists for my kids parties I told her because they are children and there is different rules for kids and adults. I then said I'm not posting about gifting her anything.

I get this is her only child and she doesn't have grandchildren however her grown child isn't my child for starters she's older then me.

I don't even want to throw this party and am regretting everything. There is so much pressure to have everything right. I am over extending myself as it is. After finding out I had not planned to gift my cousin anything other then the party I'm hosting my mom ordered her a gift from my family to her. I think this is nuts. I don't understand why this grown adult needs a party or gifts for turning 40? Is my thinking off on this or is this more common then I'm aware of?