Alright, so I checked phanna/scotu's guide on hitstun (sadly incomplete, but it had Sheik's information thankfully) and this is what I found after scotu directed me to it so nicely...
Hitlag / Shield Stun / Move
07 / 15 / Neutral Air
06 / 12 / Neutral Air (stale)
So... if I'm reading this correctly, when Sheik's Nair hits a shield, there are 7 frames of hitlag (assume full power) so that would be when the game sort of stalls. During that time, shield stun (15 frames) is occuring. So they occur at the same time (I'm just piecing this together with Scotu's shield pressure guide in mind).
So it would resemble
Frame / action
(Red is the opponent NOT in shield stun, Green is the opponent in shield stun; yay for ripping scotu's format)
1 Opponent in shield
2 Sheik's Nair hits shield at lowest point, hitlag
3 Hitlag
4 Hitlag
5 Hitlag
6 Hitlag
7 Hitlag
8 Hitlag
9 Sheik lands, L-cancel for 8 frames (1)
10 L-cancel (2)
11 L-cancel (3)
12 L-cancel (4)
13 L-cancel (5)
14 L-cancel (6)
15 L-cancel (7)
16 L-cancel (8)
17 Sheik grabs (1)
18 Grab (2)
19 Grab (3)
20 Grab (4)
21 Grab (5)
22 Grab (6)
23 Grab Hits (7)
Alright... so taking all that into consideration, I think I've calculated this correctly (this assumes you do it frame perfectly and hit their shield at the lowest possible point with the strong, not-stale Nair) then your shield-grab should tie with the opponent's at worst, and probably win if you have a better controller port, or they didn't shield-grab frame perfectly.
That's actually pretty crazy. I might ask scotu or magus or something to verify if I've done this correctly though, 'cuz I'm not sure if I made mistakes or not.
Shield stun is so weird lol.
Anyways, that looks like it works from here. At worst your shield pressure ties with them instead of giving you a free grab. And that assumes you mispaced. I bet you could space to avoid their shield-grab and have yours win because of Sheik's super grab range ****.
That's horrifying LOL. I love you.