IWB or any "close in tight" belt holster generally won't work well for a lady if carried at the side or too close to the sideline. It's a basic "shape issue". So to carry IWB ("Inside WaistBand") she has to bring it further back, which is fine as long as it's still to one side of the spine.
Follow? Cross-spine is BAD. Fall on it, or certain types of car crashes, and you risk serious spine damage.
Whether or not this will work for a given lady is all about her size and...eh..."hip width"
. The relatively low "height" of the SP101 (bottom of grip to top of rear sight) and shortness helps in stashing it in this limited "window" where it's still "rear mounted" but not across the spine. The 99 might not fit there.
Guys have an advantage in IWB because we can run the gun closer to the side without the butt jamming into our lower torsos. We can carry bigger guns IWB, as a general rule. I usually say that you can carry a snubby wheelgun with a 3" barrel just as easily as you can a 2" but with ladies, that may not be the case due to the smaller "comfort/safety window" on the belt.
I'm not ruling out IWB for either gun on ladies, I'm just pointing out an issue.
A lot of ladies run fanny packs, which can hide a 5" 1911 no sweat.
I think a better solution for ladies *must* exist, we just ain't found it yet. Basically, the gals "dip in" just at and above the beltline; use a rig designed for a guy at the side and it's a mess but we should be able to *use* that shape as a place to hide the dang thing. I'm thinking something with the butt up and forward, barrel horizontal and pointing to the rear, trigger pointing straight up and the barrel parallel with the belt and about 1" or 2" above the belt, with the anchor for the whole thing something like a padded thick leather "paddle" down the pants but anchored to the belt. You could then tuck something thick like a sweater over the whole gun just like guys do with IWB.
<scratches head> Dunno, it's just a concept at this point. Should give interesting properties though, as long as you kept the barrel at 2" for wheelguns or 3.5" for slideguns.