It is hard to keep a straight face when two sisters are looking at urinals for their dad and one sister says to the other, "I don't know if it will fit".

I started to giggle and realized what I was doing and bit my lip. Fortunately, they both saw the humour in the statement and started laughing too. What she was meaning is she didn't know if he would be able to use it since his stroke but it didn't come out that way.