YABU, you are still going to get the money, just in three months time, which is well before you actually "need" it. Your DH's siblings on the other hand, "need" the money now and he is willing to help them out by delaying his inheritance for a little while, why would you have a problem with that? It is a matter for the siblings to decide upon, nobody else. Yes your DH could have asked your opinion, but at the end of the day if all three siblings were in agreement as to the priority of who got the money exactly when, why do you feel you need to have a say as well? Would you have disagreed with the arrangement?
We had a similar situation when my nana died, her estate was to be split three ways - between her two children (my dad and aunt) and her four grandchildren (my brother and I and my aunts two daughters) with our money being held in a trust fund until we turned 18. When she actually died, my cousin and I were both 21 and finishing University a few months later and our siblings were 17/16. Once it got to actually sorting out the estate, they worked out what each "share" of the estate would be worth and it turned out that there was a fairly large amount of money that basically covered both of our "shares" but not enough to also cover our siblings' outright. The rest of the money was either tied up in bonds or property, so it would take time to release the value.
Our parents sat all four of us down and basically said "You'll all get the same money in the end but we want to give melj1213 and OlderCousin their share from the money in nana's account as it will go a long way to helping them pay off some of their Uni debt and give them a financial cushion for their start to post-grad life, and since the shares for you younger two have to go into your trust funds anyway, we decided that we might as well not tie up available money in those and instead will put the money in as and when we liquidise the other assets from the estate". Yes the younger two grumbled a bit but they both agreed that it was, ultimately, the fairest decision as it wasn't based on everyone waiting to getting their share at the same time purely to be "equal", but on us each getting our share based on priority of need.