We had an agreement with our first pregnancy. He wanted to find out so badly, and .I didn't, so .I told him if he let us keep the first pregnancy a surprise we could find out with the next pregnancy/ies.
If one of you doesn't want to find out, it's just common courtesy and respect to the person who doesn't want to know. If you wanted to know and he didn't, being a good partner would mean you shouldn't find out. He needs to give you the same respect. The rest of "his" family doesn't factor into the decision, they have no say.
Tell them that the ob office has a policy of not telling gender for fear of lawsuits or something. My cousin's office refused to tell her gender without a written and signed legal waiver.