It is their problem - YANBU. But you can also understand that their sons might want to help them. BUT - you can only afford as much as you can afford (in terms of both time and money) regardless of what they need. Have the parents been involved in the mooting of providing them with extra funds or is this something that the brothers have cooked up between them?
Definitely think that before you go into any meeting about this, you need to sit down and work out your own budget - not just for now but for the future and include contributions now for both university/education accounts for your children and a pension/savings for you - whether or not you actually do put them into a savings account, it is not unreasonable to expect to put something towards your futures, especially if you don't have a pension/named education savings at the moment.
You need to make a list of all sorts of different scenarios too - which might not be so nice to contemplate - so that you don't committ yourself to something that you can't get out of should circumstances change.
scenarios - sort of thing I am thinking about is:
* both parents just live for 1/2/3/5/10/etc years at home
* one parent moves into a home after xx years
* two parents move into a home together
* one parent moves into a home, other follows on later or has to move into a different home
* MIL dies first
* FIL dies first
* amount of money they have currently
* amount they can raise
* amount they are entitled to
* what they need now
* what they will need potentially in the future
* what will be the trigger points for you stopping to provide support or changing support depending on things in their life
- for you guys (and also brothers and their families - just realised you haven't mentioned if the bIL are married and thus don't have your extra outgoings?)
* what happens if one of you gets made redundant? (sorry, not wanting to imply that you will but in this case think you want to think of as many 'worst cases' as you can IYSWIM)
* what happens if both of you get made redundant
* are they basing amout you contribute on your joint income - in which case, what if you need to contribute towards your parent(s)?
* what happens when you need/want to contribute towards other things (such as the education stuff that people have mentioned, could well be other stuff that you have been saving up big time for in the future like a big holiday or something that this would mean you couldn't do?
* what happens when you retire if they are still alive and your income plummets - are they then going to expect your dc to pick up your share too
* what happens if something happens to your dh - would you still be expected to pay up?
* what happens if one of the brothers can't pay their share - would you be expected to pick it up?
etc etc but it is definitely worth thinking about and having a list of these before you go in then your dh can't get talked into something without at least some of the situations being looked at first.
There are some budgeting planners on moneysavingexpert.com that are worth checking out, for both you and your PIL.
Do your BsIL get paid considerably more or less than you? (sorry, realise I am saying you, but meaning you and your dh throughout really). Is it a way of them wanting to appear generous to their parents but getting you to foot a big chunk of the bill?
Are they expecting you to provide out of your savings or your current income?
If they are going to do it on how you budget then you need a couple of quick expensive weeks so that your regular expenditure is bigger and thus your surplus is smaller ) After all, if your PIL are making their budget on the basis of M&S prices then you should be too (regardless of what your actual reality is - but they certainly shouldn't be budgeting for M&S if you aren't!)
just realised that this post has gone on way too long and also has in haste assumed stuff about dc etc that were actually questions from others for you, so sorry about that.
but good luck - and make sure you at least have a bank account that is away from the brothers' grasp so that you can keep money for your retirement in (even if it is just the difference between your normal shop and an M&S one and a haircut a week!)