Hmm after reading about it, I have to say that I don't think it is a big deal if he foots the bill since it is his broke sister who is throwing it. It still isn't the MTB throwing it and inviting people. I am sure the only ones who would know would be the MTB, DTB, and sister who is throwing it. Strange to DD though.
That's the thing though, the sister would NOT be hosting because she is not footing the bill. The expecting couple is not to pay for ANYTHING. Otherwise it looks like they are hosting/paying to get people to buy stuff for them.
The sister should throw what she can afford. It's an honor to get a shower, not a right. If its dollar store balloons, cost co. Cake and Kool Aid then the expecting couple says thank you, slap on a smile and be gracious.
You don't deem the hostess's efforts unacceptable then bust out your American Express Platinum to pay for it yourself. That's rude.