Is it a sin for a Catholic to attend a different Christian church service?
No, it is not a sin for a Catholic to attend a different Christian church service. I have done so, for instance for funerals of friends or weddings of parishioners marrying a Protestant. However, as Catholics we must refrain from receiving their communion, because (first) they do not believe that they Holy Eucharist is the Body, Blood, Soul, and Divinity of our Lord, and (second) we are not in communion with them as far as fully sharing the same beliefs, traditions, leadership, etc. Moreover, a Catholic is still obligated to attend Sunday Mass even though he was at another service.