Looking at the natal charts, there is a reciprocal moon/jupiter conjunction that is a warm and loving contact for any two persons to have, they feel very good in each other's company.
Each of them has a Venus/mars conjunction, so they have a high amount of sexual energy, but the conjunctions are not related to each other.
Sun/mercury on the north node is perfect for a friendship, sharing of ideas. The other north node is conjunct Uranus/neptune/saturn that makes for a very complex relationship, fascinating, different, illusive and idealistic, but traditional and disciplined, very different types of energies all combined to make this a complicated relationship.
They also have similar ascendents, so a lot in common about how they behave.
In the composite the Venus/mars conjunctions show up as one, but is this really the case?
The moon/jupiter conjunctions square Venus/mars, and they form a T-square with the sun, the identity of the relationship.
So, it is a warm a loving relationship, is it something morbid? That would depend on a lot of factors that I don't think can be determined from these charts, we would need a horary about the question. Just because the aspects are there, doesn't mean one acts on them, there is always freedom of choice.
Keep in mind that currently transiting Saturn and pluto are touching the Capricorn planets, moving back and forth and activating them, as well as stressing the ascendent/desc axis.
You should also examine the relationship between your brother and his wife. Is there something that shows a crisis in the marriage, something amiss that she might be searching for and has found in this friendship ? Why does she feel the need to talk with her friend, even at 6 in the morning? Does she work, and is out of the house during the day, or is she a housewife with lots of free time on her hands?
The Uranus/North node could also indicate a friendship that came up out of no where and will end just as unexpectedly.
