OP you say that you have camping gear, then couldn't you check for a local campsite, or even post on a local FaceBook page, explaining the situation, and asking if anyone would be willing to allow you to pitch up in their garden for a night or two. We actually own a holiday cottage, and if I saw an ad like that, I would offer it to you for free under these sort of circumstances.
You have my sympathy about the flower situation, as we were in a very similar financial situation when my MIL died. My DH spoke about ordering flowers, and I very gently pointed out that we didn't have the money, but he was adamant that he was going to order regardless, there was no talking to him under the circumstances, as he was overwhelmed by his loss and in reality being very unreasonable, in spite of the fact that normally we had an excellent relationship, so I was forced to go with what he wanted, and struggle to pay the bill afterwards. I therefore think it might be the right time now, for you to get to know your in-laws, contact the older brother, tell him that finances are tight in the extreme, and ask if you could contribute what you can afford to the DAD tribute that he has ordered. He would have to be a real shit to say no, and you've said that the brothers seem close, so I feel sure if he were aware of the situation, he would be willing to do this. Perhaps your DH doesn't want to appear short of money to the family, due to male pride, but if that's the case it is really stupid, and short sighted, so in your shoes, I'd risk him being cross about you asking, rather than get into even more debt, due to his pride getting in the way.
You sound like you are doing absolutely everything you possibly can to support him OP, but please don't get into further financial difficulties, because he's too proud, and you feel uncomfortable asking his brother for help.