Well, for me it's a religious issue as well as a cultural one. My DH is from Egypt and we are both Muslim. I believe everyone is given what is "theirs", and sometimes that comes through other people, like adult children, etc. So we have sent money every single month for the last 18 years to my IL's including extra when needed. I have never set a "limit" or "boundary" on how much we send because I simply don't believe the money we send them is "ours". It's theirs. In other words, if we sent them $200 a month, and decided not to send it to them, then that $200 would not come to us. That might not make sense but that's how I look at it. When we first married, DH drove a taxi and I stayed at home. Shortly after, I had our first child. Then DH was a taxi driver, Master's student and I stayed at home with our son who had leukemia and so I spent most of my time going back and forth to the hospital for his treatments, and taking care of all of his needs. During that time, we were my IL's only source of financial support. We never had a month where we didn't have enough food, or a car to drive (even if it was old and raggedy) that wasn't mechanically dependable at least. We always had a place to live, even if it was modest and not in a fancy neighborhood. We always had clothes to wear, even if they came from a second hand store. We bought our FIRST new piece of furniture when we had been married about 10 years. My DH is now driving his first ever new car. We never had cable.
I have never regretted a dollar we sent to my DH's family. His family love me and treat me fabulously because they say that *I* was the one who made it possible for DH to send them money. Mind you, I never worked to send them money. When I worked, the money I made was "extra" so I bought stuff (like a bread machine for me and, as an anniversary gift, a platinum wedding band for DH) that I wanted to. One of the reasons my DH says he loves me so much is because of the complete and uncomplaining support I gave him while sending money to his family. He says he knows I could have refused, made his life miserable, demanded the money stay with us for our family/children but I didn't.
I've seen many women who've made this a battle with their DH's and have *never* seen it go well. Even in cases where I agreed with the woman that the husband was being unreasonably "generous" to his family (i.e. they have no real need for money, but he just wants to look like a big spender and puts his immediate family in cc debt, etc), it hasn't turned out well.
And I have to say, just so that I am sure go get good and flamed, LOL, that I personally do *not* agree that I and my children come "first" as a responsibility for my DH. I believe that my DH's parents absolutely should be a priority over me. I am young and healthy and can work to contribute to the income if I need to. Also, there are so many ways to stretch a dollar in the US and things that we don't *need*. My MIL and FIL are old and sick. FIL is senile and bedridden, MIL can hardly walk due to back issues and surgery that left her legs almost useless. She probably leaves her house twice a year, maybe if there is a huge reason. They raised my DH, worked hard to put him through school (and his siblings of course), loved him, made him the man I married and who I continue to choose to spend my life with and raise my children with. How can I be ungrateful and deny them some comfort in their old age?
There has been times in the past when MIL asked for extra, for stuff she didn't *need*, like some money to buy gifts for family members weddings or whatever and my DH would tell me he was going to say no because it would make things tighter for us. I always insisted he send it and more. I made myself an advocate for my IL's from the first day we were married, years before I met them face to face, and it has always served me well. Always.