You are brothers and you are an Uncle. Mazal Tov! Do whatever you feel in your heart you would like to do to secure the child’s financial future. That is extremely thoughtful of you. Any offers to help will always be appreciated whether or not they will be accepted.
Visiting and being there is the first step for being a part of this family. Helping in their home is the best way to go. Please do not be offended, as others said, if they do not feel comfortable allowing you to take the baby on outings. In addition it is always important to follow house rules, so wearing a kipah in their home might be something that will make them more comfortable as the child grows up because of the questions the child will have. You might have to make adjustments that YOU would not have considered before for the sake of the child, they will as well, but if you love each other there you will find a way to make this work for everyone.