Welcome back to SR.
No you are not being irrational.
How about telling your FIL and his wife to go to Alanon and then give this as your answer:
I have no desire to contribute even one thin dime to this "recovery" plan. For the past ten years (closer to 12, but I'll add up all the time he was sober and give him a little credit) I've paid with everything to try and keep our family afloat. All my jewelry, including my wedding set, anniversary ring, jewelry that other people had given the children, etc etc.. Anything of even marginal value (all the kids' DVDs, music CDs, players... you name it. And I'm sure you can.) have been sacrificed to his rampant addiction. We're emotional wrecks from living in the warzone created by his drug-induced psychosis.. It's not that I don't love him forever, or want to see him well. It's just that...I'm angry. And I gave at the office. I'm tapped out, and it's all I can do to keep it together for my kids, provide food and shelter for them, and try to save up for a car, since mine was repoed last year... Life in Hell. I don't have anything else to contribute to his cause.
You can't say it any clearer than that.
Sometimes the "til death do us part is just not feasible." Especially when children are involved. Your priorities are to your children.
As far as Shick-Shadel is concerned, who knows. 10 days initially is not enough time, and there is much work to be done following initial rehab. If he is really really serious, tell him Salvation Army has an EXCELLENT program and it is FREE.
As to you contributing to his rehab, my personal opinion is NOPE don't do it. That money is needed to keep food on the table, hot and cold running water and a roof over your children's heads.
J M H O
Please keep posting and let us know how YOU are doing, we do care very much.
Love and hugs,