I have stretched an ankle holsters outside of the retention straps, a Galco Ankle Lite for a M&P40c. The band stretched past where the velcro could get a firm grasp to secure the holster.
I ended up taking this to a seamstress to add another velcro strap, this worked out very well. The replaced the worn strip and added another strip, this doubled the secured velcro on the band. It also reduced the amount of strech in the band.
This may be the answer for your sister in law. You can pick up velcro strips at any major hobby/fabric store. Just have someone stitch it on an industrial machine.
This made the holster much more secure for the heavy little M&P for me and the holster is much more comfortable. Galco does make the Ankle Lite for the LCP, soft leather as opposed to the molded leather of the Ankle Glove. The LCP is a light pistol so it shouldn't be a problem of weight.
Cheers,
Sam