6th house is the house of maternal uncle, and 3rd house is the house of relatives, I think, so it can mean the house of paternal siblings, but I am not sure about it.
If the ascendant lord and 6th lord are placed in a 1-5-9 relation or 3-7-11 relation in respect to each other, then the relation between maternal siblings and the person will be good. If they are in a 6-8-11 relation to each other, then there will be conflicts. If 6th lord is placed in the 8th or 12th house, then there will be bad relation with maternal siblings, but if they are placed in any other house, then the relation will be good. Same rule you can apply for paternal relatives. But then, I am not sure whether 3rd house is to be considered for it or not as I have mentioned earlier.
"Go thou and follow Him, who was born and gave His life for others five hundred times before He attained the vision of the Buddha!" -- Swami Vivekananda