Oh dear. Weddings. The stress of dealing with one family multiplies when dealing with two!
I was going to suggest buying a second hand tux but I am glad to see that you have worked something out. Since this is obviously placing a financial strain on you, I would take the price of the suit rental off what I was going to spend on their wedding gift.
Where I got married, many bridal parties wore rental morning suit (ie tails and top hats and cravats). I told my husband that under no circumstances was I going to marry him wearing something he did not own. Likewise I knew my father would prefer to wear his own clothes (DH Dad's was not alive to attend.) LOL he was happy to get a new outfit that he could keep and wear again. I think it is nice to "be yourself" when you get married and I always regret seeing people who have been required to wear something they don't feel comfortable in. But weddings seem to bring that streak out in people!