My mother-in-law is never going to hear me say that I love her.
Never? Why wouldn't you tell her you love her at all?
I don't want to put myself on the line like that.
It's up to you who you decide to share your love with, but your mother-in-law may be disappointed if you don't express affection for her. It's normal for family members to love each other, even when they don't like one another. If you're not willing to tell her that you love her, you should consider other ways to express your appreciation.
I might do that. That doesn't sound as risky.
Relationships should never demand you express feelings in a way that makes you uncomfortable. You can make her feel cared for like she cares for you without words. Maybe help her around the house or get her flowers.