This really sounds like to me Bro doesn't really care, but SIL reallllly doesn't like Restaurant A, B, or C.
So since it is Bro's birthday too perhaps she feels like they have a say.
But like others say, who is paying for this? If you are, then too bad for them.
If this is a mutual birthday celebration with everyone going dutch, you have to decide if
this is more for you and DH and stick to your original idea, or if it is for everyone with a birthday aroudn then and come up with another mutually acceptable restaurant, E.
My feeling is that the suggestion for something other than Restaurants A, B or C should've come about before agreeing to Restaurant A, not trying to change things after the fact and when, it seems, everything was already in place. Also DB and SIL could've planned the event themselves and found restaurants everyone liked.
Years ago, a close friend and I wanted to see a movie. We invited CF's friend to go out to see a movie with us. CF and I wanted to see a particular movie but didn't think to mention it to Friend when we asked him to go with us. When we got to the theater, Friend absolutely didn't want to see the movie we wanted to see so we spent several minutes trying to figure one out. On our way back to CF's house after dropping off Friend, CF and I talked over what went wrong in our planning and realized that, next time, we invite Friend to see Specific Movie with us next time so his options are clear -- a) see Specific Movie with us or b) decline. Next time, that's what we did so when we got to theater and Friend tried to pull this same maneuver, I told Friend that this is the movie we invited you to join us to see, this is what we (CF and I want to see) and which you agreed to see and this is what is what we are going to see. I might've also told Friend he was free to see something else while CF and I go see the movie we planned on seeing but Friend backed down and saw Specific Movie with us instead. I think, next time, when CF and I asked him to see Another Specific Movie, Friend declined, which is fine -- he had his preferences, and we had ours and he was not to try to bait & switch us in the future.
The options OP listed seemed like they might be expensive (steakhouse, Italian, seafood), but it was up to DB and SIL to express their doubts before agreeing. I don't get this "false pride" (if that's what it was). I think it far better to admit you don't have the money for something than to agree then backpedal. DH and I have no problem admitting we don't have money for something (although that backfired one year because it compelled one of DH's aunt to run around decrying to the family we are destitute because we couldn't afford to do something -- we still don't have a problem stating we can't afford to do something...we just now know better than to let her know that; FTR -- we weren't destitute; we were saving our cash for our wedding that was a few months later so any expense that could wait until after our wedding was going to be postponed -- to Aunt, that meant we were one step from homeless).