I am sorry for your DH and you, and of course for your MIL. What a dreadful news! My opinion is very much coloured by the fact that I have recurrent pregnancy loss, and very much hope this current pregnancy makes it to the end and will result in our first live baby.
I understand the emotional need (and pressure) to go see her, but no way would I risk my baby's health for anyone. Even if it were my own mother, or my partner (baby's dad) in MIL's situation, I would not expose myself and baby to that. Perhaps that is harsh, but I see my priority of responsibility as to baby first, who is completely dependent on my personal decision-making without any choice in it. Babies with mums who catch Covid in third term do statistically have worse outcomes, in terms of premature delivery, stillbirth, etc. Please don't take a healthy baby for granted.
As others suggested, video-calls, care packages, letters, waving through a window, by all means show your support and love. Anything but physical exposure to either MIL or previously exposed extended family. When DH goes, ask if he wears a good mask, followed by ample testing.