I am sorry for you this is a nasty display of JW mentality. I am totally with BP what right does a non family member have to impose themselves into your grief at this time. I would speak to your husband and say that you find this very upsetting. It is likely the brother is angling to do the memorial but even so this is a time when the family should meet together and discuss what your FIL would have wished for the funeral. I would invite them to your home for an informal cup of coffee and cake and a chance to talk together.
JW funerals are horrendous even when someone dies who is well loved in the congregation the event is hijacked to witness JW propaganda and does not honour the person. This may be what your FIL would have wanted but this doesn't mean that the family can't have a private memorial at the burial site where you all have an opportunity to honour the man.
Your husband needs to think of you as well as his JW persona and to accept the invitation to dinner that excludes you is insensitive and cruel. In my place it would be easier to exclude the brother than a family member.