"Who's in front of your mother?"
Question words like 谁, 什么, 哪儿, etc., stay in the position that would be occupied by the answer. So, for example, say the answer to this question is 李老师. The statement would be 李老师在我的妈妈的前面, so the 谁 just replaces the "李老师" at the beginning.
A lot of people suggest dropping 的 is okay and I'm trying that to learn a more natural way of conversing. In a case like this with two 的，is it okay to say 谁在你妈妈后面? (dropping both 的)
Generally you should always keep the last 的 in the chain. That is also the case with this example sentence.