I'm solidly in the camp that no one gets to decide how you should spend your money. I also feel strongly that if I "have" to attend something then the host better check all the details with me first.
Given those two things, I think that you have a couple of options:
1. She's given you a good while to plan and save if you want to attend. However, if you either cannot afford or do not want to spend your vacation dollars in this way then decline and move on.
2. If you want to go and cannot afford to take the whole family, then maybe you just go alone or you and the two older children. DH and I have done similar many times over 32 years. Several years ago, just DD and I went to my sisters out of state wedding. Until his recent cancer diagnosis changed all our plans, DH was heading to his niece's out of state wedding alone next month. This option would be my preference as I fear that I would regret missing my sister's wedding.
Good luck with your decision!
Given those two things, I think that you have a couple of options:
1. She's given you a good while to plan and save if you want to attend. However, if you either cannot afford or do not want to spend your vacation dollars in this way then decline and move on.
2. If you want to go and cannot afford to take the whole family, then maybe you just go alone or you and the two older children. DH and I have done similar many times over 32 years. Several years ago, just DD and I went to my sisters out of state wedding. Until his recent cancer diagnosis changed all our plans, DH was heading to his niece's out of state wedding alone next month. This option would be my preference as I fear that I would regret missing my sister's wedding.
Good luck with your decision!