Learn Spanish (Latin America): Mis hermanos visitan la playa en julio y agosto. - My brothers visit the beach in July and August.

No, you don't use the article el with months when saying something happens in that month. You just use en directly before the month, so en julio means "in July". This is different from days of the week, where you must use el or los (like el lunes for "on Monday").

Could Mis hermanos mean "my siblings" if I have a brother and a sister?

Yes! In Spanish, the masculine plural is used for mixed groups. So Mis hermanos can mean a group of just brothers, or a mixed group of brothers and sisters. If it were a group of only sisters, you would say Mis hermanas.

Why does the sentence use Mis instead of Mi?

In Spanish, possessive words like "my" have to match whether the thing you are talking about is singular or plural. Since hermanos is plural, mi must change to mis.