Tier 2 PBS dependent visa extension

Hi There,
I am living in UK and got my ILR in 6/2018, my wife and two daughters joined me later and have not completed 5 years yet to qualify for ILR, there visa expires in June 2019 .
Please let me know how early I can apply for extension of their visa as my wife may need to travel soon to attend ailing brother , as We are not sure how many days she may need to stay outside UK .I am thinking of applying for early visa extension for my dependents . I am aware you can apply four weeks before expiry , wondering if I can apply now.
2, My wife joined me in Jan 2015 but for first two years ago she stayed outside UK multiple times due to issues back home .She never stayed outside UK for 6 months continuously. My question is that as per changes in calculation of absence from UK ,do I need to write in her application about these when she applies for ILR ( as Jan 2018 guidance for absence for dependents visa or visa / leave to remain extension issued before 1/2018 need not write about absence prior to 2018 - please guide me and clear this for me
I will appreciate your guidance
BW
NA