Grandmother
Grandma
Your ancestor mother isn't your mother, it's the mother before your mother, your grandmother.
そぼ
The reading for 祖 is known to you, but the 母 part is not. To remember the on'yomi reading for 母 in 祖母, just think about how your grandmother had a boat (ぼ), even if she didn't... and what an amazing boat it was. If you want, you can imagine your grandma is a boat too, but that might not be very nice.
私の祖母はいつも指にド派手な指輪を嵌めています。
My grandmother always has flashy jewelry on her fingers.