Yes, I think that is quite possible that they are related also to your father.

My husband tested moderately to a member here. At that time, the member and I both agreed it was through his father's line since there was a shared ancestor in the trees. When the member's parents tested, the father matched less strongly and the mother tested equally strongly to the member. When I was finally able to compare on gedmatch, the match from the member was through his mother as they both shared the same chromosome segment with my husband (although my husband did also match the father but on a different chromosome altogether from the member and his mother).