Yes, they are brothers. I found their baptisms in the same register of Poitiers Saint-Didier (BMS 1684-1692), along with two sisters. Their parents are Jean François Portal, écuyer, sieur de Lansisse, and Françoise Grimaudet.
Pierre Guillaume, born 25 Dec 1685, image 37/145
Marie Françoise, born 18 Feb 1687, baptized 19 Feb 1687, image 63/145
Jeanne Magdeleine, born 31 May 1688, baptized 1 Jun 1688, image 86/145
Henry, born 8 Jul 1689, baptized 11 Jul 1689, image 104/145
Note that their last name is spelt Portal in the records, and Portail in the index. And Henry's first name is also spelt with a Y in the record.
They were baptized in a catholic church (despite what is written on Pierre Guillaume's profile), but in those times (that was right after the edict of Fontainebleau) there was no other choice, so it does not contradict the fact that they later fled to England as huguenots. The register is full of abjurations, which were probably not all spontaneous and sincere.
I don't know what happened to Jeanne Magdeleine and Marie Françoise, but at least one younger sister, Françoise, remained in France. She died on 2 Oct 1702 at age 4 and was buried on 3 Oct 1702 in Dissay (20 km north of Poitiers), BMS 1697-1705 image 57/95. The record states that "lequel dit sieur Portal [son père] est sorty du royaume en haine de la Religion" (her father left the kingdom out of hatred of the [catholic] religion).
