Translation:Who's in front of your mother?
Damo is correct, 的 can always be omitted for a family member, but also remember that a word modifying an adjective (like the 你 modifying 妈妈 here) is actually an adverb, and 的 only ever follows adjectives or adjectival phrases. So we should here omit the first 的, and instead make it ‘谁在你妈妈的前边？’. (ps talicueh, the ellipses in 汉语 uses normal dots: ...)
@Multi - But you are NOT speaking to your mother; which is what your sentence means. "Who is in front of you, mother?" As such it cannot be considered a typo. If I was translating "I am a judge", and typed "You are judged", it should be considered wrong instead of a typo. If I am translating "He is correct", and I type "He is incorrect", it should be considered wrong instead of a typo. In short, typos can be considered only when the mistyped words don't completely change the meaning of the sentence. That's what I think. Happy to hear others' thoughts.