There are traditional Orthodox Christians who would say that if we are "Not to pray with heretics" (or is the quote, "heathens"---I can't recall which), our canons would preclude us from attending non-Eastern Orthodox church services. It is not my intention to refer to Roman Catholics or other Trinitarian Christians as "heretics."
Not that I am a canonist by any means, and not that I look to a modernist spin when examining traditional Orthodox practices and canonical directives, which I believe compromises our faith, but in this type of a matter, I feel, during the first millennium, the Holy Fathers did not contemplate Orthodox Christians living as a minority community in a pluralistic and relatively secular society; not contemplating that within our own immediate families, we would have non-Orthodox Christian relatives, and friends for that matter.
I see nothing wrong with a devout Orthodox Christian's attendance at a non-Orthodox religious ceremony, particularly, the funeral of an immediate relative. In fact I would find non-attendance at a parent's funeral, most ungrateful, disrespectful, and rude. As some guidance above suggests, we do not participate in these ceremonies, especially in heterodox sacraments. (Frankly, I try to avoid shaking hands at Roman Catholic services, by keeping my head down, though there are those ever friendly Roman Catholics who will go out of their way to seek me out, in which event I relent.)