You did do what was right at the time and the place. DD being furious with you, at a time when a branch of your family tree was disintergrating after 15 years before your eyes, was not a position for you. DD considering a relationship with you as collateral damage while cutting off her ex, seemed too viable a possible outcome when things were that volatile. Your grandchildren's relationship with you being stable was good for them. It still is.
While financial, custody, and possible issues are being battled, even seemingly innocuous information, like answering "Where's DD today?" can have legal consequences. While it would have been wonderful if you could have consoled SIL during that initial period, and he you, there would have been so many truly hidden landmines at that time. Any accusations of you passing information to him would be difficult to refute. Whenever DD wondered, how did ex find that out about me?, it would have been (mis)credited to you. I think SIL, like you, understands all too well what DD was willing to do when furious at that point. I think SIL is still be open to your love and understands why you had to pull back. If time heals and DD eventually relaxes her battle stance against him, and does not see her ex as her enemy...she may not always feel that relationship is you fraternizing with the enemy.
You made a sacrafice but for that time and that place, it was a wise one.