Your fears that she will 'steal' them are irrational, as you know - but those fears will be based on some very rational feelings and experiences.
Will be? I think you meant "could be" @BatsInSpring
My sister is constantly, uerringly, the same about her DC as OP has described how she currently finds herself. Without knowing OP, & her history with SiL, we don't know why.
I hope her fears ARE irrational, & that she can reframe & reconcile them.
Or if they are rational, that she follows your otherwise sensible advice.
In my sister's case, her fears are based on lack of object permanence, enmeshment, possessiveness, & downright coercive control. It doesn't stop her from totally rationalising them - to her own satisfaction, if not logic's! - & acting as if she is not just entitled to feel this way, but other people are actively undermining her if they disagree.
Had she posted here, & read your response validating her batshittery unfortunate controlling & possessive nature as being rational, she would have taken it as permission to adopt your advice as a Flying Monkey to beat anyone who would listen over the head with.
Having said that, OP isn't coming over as controlling - more worried, & perhaps a little dispossessed. So I've given her gentler & I hope much more thoughtful advice upthread. Had my sister ever asked me she'd have had a more robust response. Certainly not associating OP directly with my sister's unpleasant mentality, but it's not the best bet to advise anybody that what are most likely irrational fears are based in concrete reality.