Pleading the 5th is just a legal thing. I think that in this situation it's being advised by their lawyers in order to defer any discussion of those questions until criminal trial for the charges against them for Sherin.
I don't completely understand the legal implications of pleading the 5th, though, because to a non-legal person it sounds very much like admitting that they've got something to hide. Invoking the 5th is surely to say "if I answer this I risk implicating myself", and we 'know' it's not that they're going to implicate themselves in something unconnected.
After hearing the last Maria Guerrero FB live I am torn about the milk story. I am going back to something I wondered earlier, and that is whether Sherin was fed something to make her choke, something gritty in the milk? But in the Maria Guerrero Q&A FB Live, she said something about WM giving Sini the choice to either go out to dinner as a family or for Sini to stay home with Sherin? Does that imply that Sherin was alive and either well or sick at that time? And the 'new' timeline of 10pm to midnight for WM trying to get Sherin to drink her milk, shouting at her and Sherin taking a sip each time she was shouted at and procrastinating in between being shouted at. I can imagine that scenario happening in Sherin's life, but I don't know if it happened that particular night or if he's recalling a situation that happened another time to try to explain how Sherin died during a fight for her to drink her milk. That story also makes me question whether WM is just as nasty with Sherin when he is the one doing the parenting and why that might be? Is he emulating Sini's treatment because he's following her lead, is it because he's been brainwashed to feel this is how Sherin should be treated, or is he describing fights that Sini had with Sherin and just inserting himself into the power position in order to take the blame for Sini?
The back story that's coming out in the custody hearings does not make me feel that WM is the primary abuser, but even someone who's been psychologically abused can be convinced to abuse someone else, and that can even be a part of psychological abuse, the power of forcing someone else to abuse another person can feel very 'empowering' to an abuser, especially to force them to abuse something/someone that they're attached to and care about and in the normal way would want to protect.