I think it's the best decision you can make for SD, Nik. I'm one that gives no leeway for age or blood-relation when it comes to toxic people in my life & especially in DD10's life.
In fact, I have been having very big issues with my 90-yo Gram (my mom's mom) over the last 6 months as it relates to abuse of my mother that I have only just found out about. It's a very long story, but the relevant part is that I told my mother in no uncertain terms that I may very well never see or speak to Gram alive again at this point & I'm ok with that. She is a very sharp, together woman who has amazing health for her age but her toxicity to those around her & the way she is choosing to handle this family crisis is something I won't make excuses for nor will I look the other way while she feigns memory loss or blameshifts. She is avoiding me knowing that I will call her out but trying to manipulate my mom into setting up for her to be able to visit with DD - no way, uh-uh, ain't gonna happen on my watch.
Toxic is toxic & there is no room in my life for that...... since I'd rather save for DD's college vs. therapy fund - there is no room for them in her life as long as I have a say-so.