...You've pretty much just demonstrated that you personally don't know how to handle either of those MUs, not that they are actually bad. No offense.
In fact all you really did here was list Sheik and ZSS's generic strengths (Sheik has superior frame data to everyone in the game but that on it's own isn't everything, for example) and assume that Rosalina can't handle them, rather than actually give any particular reason why Rosalina would struggle against them.
Rosalina can actually rival Sheik's frame data because Lunar Landing aerials is a thing and those can immediately be followed up on by Rosalina, especially if Sheik actually goes to hit Luma, and most of Rosalina's (and some of Luma's) moves literally go right through Sheik's anyway. In particular, throwing out N-air for days works wonders, since that move has very little landing lag and covers Rosalina from all sides with Luma around. Also needles are not effective against Rosalina, Luma can wall them out, they can be GP'd, and they actually have quite a bit of end lag so she can't follow up on Rosalina GP'ing if she's too far, and Rosalina can actually punish Sheik for using needles if she is close and lets Luma sponge the needles. So Sheik's zoning game (actually one of her biggest assets, it's largely what helps her get in at characters), is pretty much useless in the MU. Against a character that outranges her and can rival her frame data with various tricks, and a character she has to really work to get in at, it's difficult to say Sheik can have the advantage here. It could be considered even but if any character has the slight advantage here, it's easily Rosalina. Rosalina just has all of the defensive tools she needs to deal with a Sheik onslaught, and half of the time that doesn't matter as Sheik has trouble getting in at Rosalina to begin with, due to Rosalina outranging her and always having the ability to have a hitbox out that beats any of Sheik's. And of course, being able to KO Sheik way earlier and easier than Sheik can KO Rosalina, is a big plus for Rosalina too. It's not an easy MU for Rosalina per se, and I can understand why people might have trouble her as Rosalina, but if you know what you're doing, Rosalina really has the upper hand here.
ZSS is more or less the same deal only she has even less going for her. ZSS's paralysis shots can not only be GP'd, but their hitbox actually remains active and then switches ownership to Rosalina. This means that once again, ZSS's projectile is useless and in this case, can actually backfire if she goes in for a punish in a lot of cases, and it actually paralyses her. ZSS also has no truly effective means of shield pressure, and her grab is easy to read and punish. ZSS's Down B is perfectly countered by Rosalina shielding and U-tilting too, especially with Luma. I also find it's easy to mess ZSS up by separating Luma often and quickly and intelligently maneuvering him about so he can punish her and be ready for combos, since she's going to attempt to space you out a lot, especially with Z-air. And in general, a well spaced Luma will do way more for Rosalina than any of ZSS's tools will for her.
I could go way more in-depth, especially on ZSS, but in general MU discussions aren't my thing anymore since I know how the players on the Rosalina boards play way too similarly to each other with a limited play style (thus they don't play with he kind of play style I do), and in general the attitude of non-Rosalina mains always saying their character is an underdog against her being a prevalent one. This is also the social thread, so...