My DD1 and her best buddy are on 11th & 19th Feb so we have always had a combined party for them - now DD2 was also born on the 19th Feb so we will be doing a combined 4yo and 1yo party this time.

Best buddies sibling is due 2nd March so the following year we will add him/her into the celebrations.

I think it takes pressure off people having to attend more than 1 party and you also only have to organise one event.