Hello! Thanks again for all the replies!
So I should clarify, I met DH when he was a student, and he wasn't expected to pay for this reason. For five years, he was studying, while I worked full time, and so didn't pay. Had children (very small age gap) after this and wasn't in a position to pay because money was very tight. However, the expectation is that we will start to pay soon.
For those of you who know about Indian traditions, you're right. It's NOT tradition for the daughter to live at home when married. She should be living in her husband's home, according to the tradition. So those of you who have noticed that they're willing to break tradition for her, have hit the nail on the head. And actually, now I think about it, I guess this is another part of the reason that we are reluctant to stick to traditions - not only that we can't afford to, but that maybe the 'tradition' card is played when it suits them.
To the poster who said that SIL 'looks after' PIL instead of us doing it - this couldn't be further from the truth! (DH laughed out loud when I read that to him!) SIL is lovely, but she is very, very lazy. She doesn't like her PIL and doesn't want to live in their house because they expect her to help with housework, and they won't look after her son for her. When SIL is at her parents house, she is like a queen - she doesn't have to life a finger. Her DC is looked after 100% by my MIL (even sleeps in the room with her at night so SIL doesn't have to get up for a night feed) and SIL doesn't cook, dress, play with or take care of the baby in any way. When she stays with her parents, they give her spending money to go out shopping, for meals, for movies. She doesn't work. My PIL have just paid for her to do a course she wants to do, so she's doing that for now part time, but mostly living a life of luxury! PIL are very much looking after her, and not vice versa! They're not elderly, they're all fit and active (PIL are in their fifties) and they're really not in a position to need 'looking after'.
Her husband is financially abusive, definitely. But again, this is partly because of tradition. SIL is actually 'humiliating' her DH and his family by refusing to stay in their house, and so her DH (as much as I can't stand him) is, in a way, in a horrible position. He is refusing to send her money because he's also under pressure from his parents to encourage her to stay in their home more. I don't agree with his behaviour at all. But I don't think it's fair that SIL is encouraged to break tradition (PIL baby her a lot and can't bear the idea of her having to help out with any housework, so they encourage her to stay at their house where she will be 'treated properly'), her DH's family are expected to put up with her breaking tradition, and then at the same time tradition is used to make us do what they want us to do, if you see what I mean.
As another poster said, no, traditionally the daughter would not be given anything by the parents in their will. The son would financially support the parents and then he would inherit all land. But this is another tradition which they are breaking (they have always said that SIL will get 50 percent of everything). Which we don't disagree with at all! And like I said, we aren't really interested in the inheritence anyway. But I just mention this to show that this is another tradition they're breaking in order to benefit SIL.
I love SIL and I feel awful for her - the arranged marriage is horrible. She's just not at all compatible with her husband. But PIL are willing to break all traditions to make her happy.
They regularly complain about her husband - that he won't agree to move out of his parents house to build SIL her own house where they can live alone (which it totally against tradition), that he spends money on his own family (they think he should spend his money on SIL and DC and not his family), that he and SIL can't afford to travel etc because he puts money towards his own family home. In one breath they're saying this, and saying that he should break traditions because SIL deserves better. But in the next breath they're asking DH and I to give up everything, leave our lives behind, and to come and live the traditional life that they think SIL is too good for.
So PP's are right who have pointed out that they expect us to give up our quality of life to serve them in the 'traditional' way, but that they don't want this for SIL, so it is quite hypocritical.
I just read this post back and it's not meant to sound as bitter as it does! But those of you who have pointed out the differences between how they treat SIL and how they expect us to behave have really hit the nail on the head. PIL have never given us anything, and we don't expect anything. But they have given SIL a huge amount of money and continue to do so, while at the same time expecting us to send money back which would probably be spent entirely on SIL now - on treats to cheer her up or on courses she wants to study or trips she wants to go on, because her DH won't pay, and because she doesn't like working - or which we will be given back 50% of in the future, while she is given the other 50%.
I also understand what those of you are saying about not wanting to sell the land because it might be ancestral land or have emotional value like that. However, FIL told SIL's husband that he would sell the unused piece of land if SIL's husband would use the money to build her her own house, so she doesn't have to live with his parents. (But he has flat out refused to move out of his parents family home, much to PIL dismay.)
So it's not all as simple as it seems, or as I maybe made it sound! Sorry for the huge drip-feed. I didn't realise this would be relevant to my original AIBU but I guess it actually is, very much so!