Shared birthday party

Okay ladies, my daughter will be turning 3 in July and my niece will also be turning 3 in June. They are exactly 1 month and 1 day apart. My sister has been wanting to do a shared party for them and I finally agreed to it this year. So my question is, would you fill obligated to purchase a gift for each birthday girl ? It will mostly be family anyway but we each have a friend or two that will be invited. I don’t want our family or friends to feel obligated to purchase ANY gifts but especially since it’s a shared party. Also, would you like the idea of having a shared party to knock out 2 bdays at once? Also, what’s your ideal time for bday party your invited too? Early afternoon, midday or late afternoon?