I patronize my husband's parents although his brothers and sisters don't offer any help for them; Yet, they keep accusing me of being the reason of any dispute with their father. What should I do in this regard?
All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
The wife isn't obliged to patronize the parents of her husband save if she volunteered to do so and will be rewarded by Allah, The Exalted.
In addition, Allah forgave a person for removing harmful objects from the road and rewarded him through entering paradise, since Abu Huraira reported Allah's Messenger (PBUH) as saying that he saw a person enjoying himself in Paradise because of the tree that he cut from the path which was a source of inconvenience to the people. [Related by Moslim]. Therefore, patronizing human's is more prudent in this case as well the person doing so will have great reward by Allah.
Thus, we recommend you (Questioner) to be more patient regarding the accusations of your husband's brothers and try to clarify the truth in each situation and if this didn't work, then you should avoid them as possible and keep patronizing his parents as Allah will suffice you. And Allah knows best.