"Does your mom have brothers?"
Translation:У вашей мамы есть братья?
Here is an explanation bordering on major oversimplification which may help you:
When questioning something's existence, then use У ... есть.
- У вашей мамы есть братья? (Implication of the question: I do not know whether or not your mom has brothers.)
- Да, у неё есть братья.
On the other hand, when needing qualification rather than yes/no, use У ... without есть; it assumes that the asker already knows something exists, and does not wish to confirm existence with есть, but rather wishes for a description rather than a mere yes/no.
- У вас яблоко? (Implication: I know you are holding something behind your back. Something exists back there. I bet it's an apple, right?)
- У меня торт! С днём рождения!
- Шоколадный торт! <3 <3
See also @olimo's always super helpful helpfulness above - she goes into what the implication would be of using У вашей мамы братья without есть.
What I find with Russian is that it does not translate 1:1 with English; using certain words such as у ... есть and using perfective and imperfective verbs (if you haven't gotten there yet, don't lose heart once you do; bookmark Russian for Everyone and Russian Lessons for future use) carry nuances of meaning, rather than allowing for any sort of direct translation.