I've been married to my husband for 6 years now, and every year my MIL&FIL come to the US to stay with us for a month. Each year we pay for their whole stay (flights, housing, food, special trips, gifts, etc) this year my SIL will be traveling and my husband wants to pay for her flight too. He hasn't seen her in over 30years, but I told him no. I told him if she wants to come she needs to pay for her flight, to which he replied, "if this was your siblings and you haven't seen them in a long time, you'd want to do it too" am I overreacting? I know we can afford the flight but the day my husband will go over there to see them NOBODY is going to pay for his whole stay abroad. How can I make him understand this??

Edit:

SIL is married with kids. Family of 4 was supposed to travel originally but their plans changed and she will be the only one coming with my in-laws. This makes me assume they've considered their considered paying for their own flights otherwise why would they come? So now that's she's traveling by herself why should we pay for her flight?