In my opinion it's how they play the courtroom game of getting something in that wouldn't normally be allowed in, and it flopped in any event.
You want to establish that if LV wouldn't confide in the brother who followed her from State to State like a lamb and even gave up his job to be live in the house next door to her, she is unlikely to have told anyone at all. So because brother is dead you establish a sister who can give an opinion on how LV would treat people as close as her brother. That's why, imo, he tried twice to get MG to say they were as close as sisters, the second time showing his frustration with her when she still wouldn't give him the answer he wanted. "Is there a reason why you're not saying yes?" - asked with a snarl.
I imagine he would have replied to the objection with something along the lines of 'I think I should be allowed to ask the surrogate 'sister' her opinion since the equally close brother isn't available to the defense'. It was a long build up to trying to get her to qualify on the stand to give that opinion.
MOO