I like flashingnises's suggestion - but may word it more storngly. Ring her and say you are sorry to hear that she/they are having difficulty in bufgeting for the wedding - and that you can fully sympathise, as you yourselves have to work hard to budget for your own outgoings and never have anything spare. Make a pointed comment like, "I suppose ti is all a matter of priorities - and at the moment ours are the kids, and over the next year, yours will be the wedding" - and leave it at that. Just state it as a fact without any hint of judgement
Don't even suggest that yuo could contribute if you had more money. If you really want to lay it on the line, who could hint at how much of a struggle it will be even getting the whole family over to Dublin for wedding.
That way, she then knows that you cannot afford - and have no intention of doung so anyway - to contribute to the wedding - and will only embarrass herself if she tries to bring it up (if the usggestion came from her) or she can make it clear to her mother that you are not an option! (Doesn't help your other relatives though, if they are being asked for contrivutions too!).
And iether don't tell your FIL - or, ifit's goiung to get back to him anyway, pre-empt it and let him know that you (and your dh - his son after all) think that that the sister is being totoally out of order and that he has your full support for him to contributre wahtever he would like to and can afford. At 30, SIL should be prepared to stand on her onw two feet - or, if she wants to go with the traditional "her parents pa", then have the maturity to sit down with bboth parents and work out a budget that can be afforded.