When I threw a surprise bday party for my (then) DHs 30th, I was 8 months pregnant. The party was held at DHs hometown (2 1/2 hours away from our house, but down the street from his parents, sister and brother's places.

No one lifted one finger to help me pick up food and drinks, decorate, or arrange a single detail. I hinted I needed help but did not ask for help outright. No help was given. Part my bad (for not asking), part theirs for knowing I needed help but not offering an 8-month pregnant woman any help. I didn't ask for financial help, and that was ok by me.

Don't expect squat from them.

P.S. Later I made sure DH knew how much fun and how much work I put into making his day special, and hoping he enjoyed it as much as I did. He assumed his family helped me. I made sure he knew different.

It's all on you financially. If anyone else offers that is a bonus.
However, I think it is permissible to ask family members for help with decorating, specific tasks or keeping him busy while you work on it so it stays a surprise. BUT you can't get mad if someone says no as it is not required.

On a side note, I was at a 50th birthday party recently where a sibling gave the other sibling a getting old kit. It include Depends, gas relief pills, denture cleaner and cream, reading glasses, arthritis rememdies, nose hair trimmers, penis pump, cane, etc.
It was hilarious!

I get the feeling you feel, generally, slightly let down by the in-laws-to-be. That can be particularly difficult for SMs because the in-laws-to-be are often less accepting/welcoming of us than they were of the first wife, not because they preferred her, but somehow because second-time-around it feels less significant to them. We have a harder barrier to cross to get involved with the blood family.
In my view you might be better off throwing a smaller party for a circle of friends rather than family. Make clear it is not a snub to family but it is just one of the many celebrations he may like to have and you're thinking of his friendship rather than his family network. That way you may feel happier about and hosting all the guests, without the feeling of resentment that you are developing for having hosted gatherings for a group of people who appear not to be reciprocating or appreciating in the way you would like.

I had my DH a big surprise party in our back yard when he turned 40. I was fortunate enough to be able to afford to have it catered, so I didn't expect or really want my MIL or DH's family to help pay for it, but if it would have been something that DH and I could not afford, I wouldn't have minded asking MIL or DH's sisters if they would be willing to co-host it (meaning co-sponsor!) MIL's main role was to help make the guest list with friends from DH's childhood and track down their addresses for me (and then hound them by calling them and/or their parents when the RSVPs were not returned, a MAJOR problem we ran into.) MIL also made him a cake, which she offered to do and wasn't necessary because I also ordered a dessert with the meal. The main "help" I needed was getting tables and chairs that we rented for the party, which my mom and best friend went with me to pick up and set up, as well as, decorations--I didn't ask DH's family for any help there, but again, it wasn't needed. Had I needed help, I would not have hesitated to ask.

Is it possible your MIL/BIL do not think you need any financial assistance with it? OR, do you think they are not able to contribute financially? If you think they are able to help out, I would not hesitate to see if they are interested. If you don't feel comfortable being direct, maybe you could even just say that you want to do something but can't really afford to do it on your own, so you're not sure what your options are. And maybe if they don't help out, you can still have him a surprise party but keep it to his closest family and friends, which would still make it special.