If the aunt just wants the weekend, that seems reasonable. There should be no stressing on your break. Seeing as you have 5 weeks off, it seems reasonable to go to the aunts for the weekend. Speak to your parents, I'm sure they'll understand.
I did not fully understand your post but maybe you can tell your husband how excited your siblings are about you two visiting the town and have made plans for a weekend or two for some family fun.
I dont know how important the aunt is and why he he chose to live at hers over instead of your parents especially when your family is excited about your visit.
we have found a way of splitting the 4 weekends...which means i will need to take annual leave in order to have some time alone with the hubby in London