ならまだしも is a combination of なら 'if', まだ 'still', and the
adverbial particle しも, which has the primary purpose of strengthening the word that comes before it. When used, this structure will often be translated as '(A) is fine, but not (B)', or '(A) is fine, but it's different when it's (B)'.
ならまだしも is a phrase that usually separates the first and second half of sentences, so will be attached to the end of the (A) phrase, marking the statement that is acceptable.
-
暑（あつ）いだけならまだしも、湿度（しつど）も高（たか）いからどこにも行（い）きたくない。
If it was hot, that would be fine, but because it is also humid, I don't want to go.
二（に）、三日（さんにち）ならまだしも、二週間（にしゅうかん）なんて待（ま）てませんよ。
If it was 2 or three days, we could wait, but 2 weeks? We can't wait that long.
一回（いっかい）だけならまだしも、毎日（まいにち）同（おな）じことで怒（おこ）られて何（なん）にも思（おも）わないの。
If it was only once, okay, but don't you feel any sort of way being told the same thing everyday?
Due to the way that しも alters まだ, a more literal translation of this grammar point can be thought of as 'if it's (A), then of course there's still that, but as for (B)'.
しも is a classical particle that is not frequently used in modern times outside of set phrases or expressions like
ならまだしも, 今（いま）しも 'right this moment', and だれしも 'absolutely anybody'. In all cases, it merely strengthens the word directly before it.
-
今（いま）しも家（いえ）を出（で）ようとしていたところだった。
I was just about to leave the house.
誰（だれ）しも長年（ながねん）使（つか）ってきた釣（つ）り竿（ざお）には愛着（あいちゃく）がある。
Everyone has an attachment to a fishing rod they have used for many years.