I think it is also fine to leave out the 的 when talking about one's own family members. 我妈妈，我弟弟，我哥哥, etc should all be fine to say as what I've learn from my classes.

I'm a bit confused, the explanation said whenever I use "this" or "that" I should add the measure word "gè", any help? Also maybe it's my bad ears but I hear the tone as "ge"

D'oh. Possessive on family members is supposed to be optional.

I also think the same , I have heard 这个 我 的 妈妈

the possessive particle can be dropped when talking about one's own family, this should be fixed